xtokens: ensuring exact error from xcm propagated

[dzmitry-lahoda]

xcmp may fail because of dozens of reasons. it is very hard to debug and hard to setup.
so solution is to be very specific about failure reason.

[xlc]

I am fine with this. Later can be updated once paritytech/substrate#10242 is available

[codecov-commenter]

Codecov Report

Merging #659 (351ba2a) into master (03fe554) will decrease coverage by 0.27%.
The diff coverage is 16.66%.

@@            Coverage Diff             @@
##           master     #659      +/-   ##
==========================================
- Coverage   75.93%   75.65%   -0.28%     
==========================================
  Files          78       78              
  Lines        6549     6577      +28     
==========================================
+ Hits         4973     4976       +3     
- Misses       1576     1601      +25     

Impacted Files	Coverage Δ
xtokens/src/lib.rs	59.76% <13.79%> (-9.75%)	⬇️
xtokens/src/tests.rs	100.00% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 03fe554...351ba2a. Read the comment docs.

[dzmitry-lahoda]

replaced with logger. added comment to use paritytech/substrate#10242 as soon as ready.

[zjb0807]

Suggested change

	log::trace!("Failed execute transfer message with {:?}", error);
	log::error!(target: "xtokens", "Failed execute transfer message with {:?}", error);