[improve](load) improve error messages for "no backend available"

[kaijchen]

What problem does this PR solve?

Issue Number: DORIS-12832

Problem Summary:

Improve error messages for "No backend load available." and There is no scanNode Backend available.
Suggest users to check backend status when the error occours.

Release note

None

Check List (For Author)

• Test

  • Regression test
  • Unit Test
  • Manual test (add detailed scripts or steps below)
  • No need to test or manual test. Explain why:
    • This is a refactor/code format and no logic has been changed.
    • Previous test can cover this change.
    • No code files have been changed.
    • Other reason

• Behavior changed:

  • No.
  • Yes.

• Does this need documentation?

  • No.
  • Yes.

Check List (For Reviewer who merge this PR)

• Confirm the release note
• Confirm test cases
• Confirm document
• Add branch pick label

[hello-stephen]

Thank you for your contribution to Apache Doris.
Don't know what should be done next? See How to process your PR.

Please clearly describe your PR:

1. What problem was fixed (it's best to include specific error reporting information). How it was fixed.
2. Which behaviors were modified. What was the previous behavior, what is it now, why was it modified, and what possible impacts might there be.
3. What features were added. Why was this function added?
4. Which code was refactored and why was this part of the code refactored?
5. Which functions were optimized and what is the difference before and after the optimization?

[kaijchen]

run buildall

[doris-robot]

TPC-H: Total hot run time: 32240 ms

machine: 'aliyun_ecs.c7a.8xlarge_32C64G'
scripts: https://github.com/apache/doris/tree/master/tools/tpch-tools
Tpch sf100 test result on commit 779eec94aa317380c5fc26fa4a52dda9c0992682, data reload: false

------ Round 1 ----------------------------------
q1	17595	5493	5395	5395
q2	2044	312	180	180
q3	10403	1279	727	727
q4	10215	974	511	511
q5	7521	2407	2205	2205
q6	194	170	140	140
q7	891	770	613	613
q8	9235	1362	1113	1113
q9	5231	4856	4869	4856
q10	6862	2344	1905	1905
q11	457	277	268	268
q12	339	363	215	215
q13	17780	3711	3147	3147
q14	233	231	209	209
q15	516	475	475	475
q16	625	633	597	597
q17	554	857	318	318
q18	7011	6358	6504	6358
q19	1221	956	539	539
q20	310	327	189	189
q21	2819	2180	1955	1955
q22	365	336	325	325
Total cold run time: 102421 ms
Total hot run time: 32240 ms

----- Round 2, with runtime_filter_mode=off -----
q1	5496	5510	5509	5509
q2	240	327	229	229
q3	2271	2682	2333	2333
q4	1388	1873	1351	1351
q5	4265	4778	4694	4694
q6	165	159	129	129
q7	2056	2016	1798	1798
q8	2643	2834	2689	2689
q9	7246	7176	7196	7176
q10	2998	3219	2932	2932
q11	597	529	518	518
q12	706	780	632	632
q13	3536	3914	3303	3303
q14	270	313	287	287
q15	538	474	462	462
q16	638	701	663	663
q17	1212	1737	1258	1258
q18	7619	7298	7246	7246
q19	797	1074	1101	1074
q20	2027	2019	1881	1881
q21	5764	5320	5034	5034
q22	591	584	585	584
Total cold run time: 53063 ms
Total hot run time: 51782 ms

[doris-robot]

TPC-DS: Total hot run time: 184668 ms

machine: 'aliyun_ecs.c7a.8xlarge_32C64G'
scripts: https://github.com/apache/doris/tree/master/tools/tpcds-tools
TPC-DS sf100 test result on commit 779eec94aa317380c5fc26fa4a52dda9c0992682, data reload: false

query1	969	385	386	385
query2	6524	2025	1983	1983
query3	6789	215	238	215
query4	33464	23270	23211	23211
query5	4318	621	500	500
query6	308	197	189	189
query7	4606	482	306	306
query8	293	240	230	230
query9	9444	2632	2640	2632
query10	486	317	276	276
query11	17945	15217	15016	15016
query12	160	111	106	106
query13	1676	522	403	403
query14	10490	6767	6873	6767
query15	233	207	180	180
query16	7980	646	462	462
query17	1608	740	570	570
query18	2103	387	283	283
query19	217	176	146	146
query20	122	112	107	107
query21	205	120	105	105
query22	4339	4308	4207	4207
query23	34134	33022	32934	32934
query24	6413	2298	2214	2214
query25	493	476	373	373
query26	785	270	159	159
query27	1997	457	326	326
query28	5276	2431	2436	2431
query29	638	549	402	402
query30	229	188	194	188
query31	963	868	775	775
query32	87	63	62	62
query33	510	361	291	291
query34	735	860	492	492
query35	833	830	724	724
query36	988	1057	936	936
query37	125	104	81	81
query38	4214	4143	4077	4077
query39	1505	1386	1400	1386
query40	201	119	103	103
query41	53	53	51	51
query42	128	107	100	100
query43	509	506	478	478
query44	1294	806	820	806
query45	179	178	162	162
query46	850	1061	644	644
query47	1806	1823	1764	1764
query48	377	400	318	318
query49	732	474	406	406
query50	632	682	408	408
query51	4214	4187	4116	4116
query52	110	99	97	97
query53	233	261	192	192
query54	471	479	419	419
query55	84	77	79	77
query56	260	266	241	241
query57	1159	1141	1100	1100
query58	255	232	238	232
query59	2985	3104	2935	2935
query60	273	271	257	257
query61	119	117	114	114
query62	800	712	655	655
query63	225	189	189	189
query64	3680	1023	639	639
query65	3256	3135	3169	3135
query66	757	422	308	308
query67	15840	15473	15295	15295
query68	2721	832	553	553
query69	450	293	275	275
query70	1237	1186	1095	1095
query71	361	288	244	244
query72	5850	3795	3855	3795
query73	638	750	369	369
query74	9727	8793	8920	8793
query75	3256	3152	2678	2678
query76	3005	1184	757	757
query77	489	357	266	266
query78	10003	9892	9505	9505
query79	2503	776	596	596
query80	1690	526	452	452
query81	577	320	237	237
query82	326	152	124	124
query83	262	185	152	152
query84	237	100	70	70
query85	799	350	295	295
query86	461	318	303	303
query87	4548	4348	4394	4348
query88	3669	2180	2145	2145
query89	375	327	291	291
query90	1636	198	194	194
query91	138	138	108	108
query92	61	56	54	54
query93	973	859	538	538
query94	666	417	308	308
query95	338	273	264	264
query96	476	604	280	280
query97	2809	2890	2742	2742
query98	229	209	204	204
query99	1286	1375	1262	1262
Total cold run time: 277482 ms
Total hot run time: 184668 ms

[doris-robot]

ClickBench: Total hot run time: 30.76 s

machine: 'aliyun_ecs.c7a.8xlarge_32C64G'
scripts: https://github.com/apache/doris/tree/master/tools/clickbench-tools
ClickBench test result on commit 779eec94aa317380c5fc26fa4a52dda9c0992682, data reload: false

query1	0.03	0.03	0.03
query2	0.07	0.04	0.03
query3	0.23	0.08	0.07
query4	1.62	0.10	0.10
query5	0.41	0.42	0.40
query6	1.16	0.66	0.67
query7	0.02	0.01	0.02
query8	0.04	0.03	0.03
query9	0.58	0.50	0.52
query10	0.55	0.56	0.56
query11	0.14	0.10	0.10
query12	0.13	0.11	0.12
query13	0.62	0.59	0.60
query14	2.73	2.88	2.79
query15	0.87	0.83	0.83
query16	0.39	0.40	0.39
query17	1.05	1.06	1.01
query18	0.22	0.21	0.21
query19	1.91	1.87	2.03
query20	0.02	0.02	0.01
query21	15.35	0.93	0.60
query22	0.76	0.78	0.67
query23	15.30	1.41	0.59
query24	2.69	0.70	1.97
query25	0.12	0.25	0.12
query26	0.31	0.16	0.14
query27	0.05	0.06	0.06
query28	14.01	1.03	0.43
query29	12.58	3.96	3.26
query30	0.25	0.10	0.06
query31	2.83	0.61	0.39
query32	3.23	0.55	0.46
query33	2.97	3.03	3.06
query34	16.62	5.20	4.61
query35	4.53	4.61	4.54
query36	0.64	0.49	0.49
query37	0.10	0.07	0.06
query38	0.04	0.03	0.03
query39	0.04	0.03	0.02
query40	0.17	0.14	0.13
query41	0.08	0.02	0.03
query42	0.04	0.02	0.02
query43	0.03	0.04	0.03
Total cold run time: 105.53 s
Total hot run time: 30.76 s

[dataroaring]

LGTM

[github-actions]

PR approved by at least one committer and no changes requested.

[github-actions]

PR approved by anyone and no changes requested.

[liaoxin01]

LGTM