fix(databases): UnboundLocalError in Exception Handler for load_engine_specs

[cancan101]

Currently you get this error if the Exception handler is hit:

UnboundLocalError: local variable 'engine_spec' referenced before assignment

SUMMARY

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

TESTING INSTRUCTIONS

ADDITIONAL INFORMATION

• Has associated issue:
• Required feature flags:
• Changes UI
• Includes DB Migration (follow approval process in SIP-59)
  • Migration is atomic, supports rollback & is backwards-compatible
  • Confirm DB migration upgrade and downgrade tested
  • Runtime estimates and downtime expectations provided
• Introduces new feature or API
• Removes existing feature or API

[codecov]

Codecov Report

Merging #19422 (112891d) into master (0968f86) will decrease coverage by 0.16%.
The diff coverage is 0.00%.

❗ Current head 112891d differs from pull request most recent head 1e80d73. Consider uploading reports for the commit 1e80d73 to get more accurate results

@@            Coverage Diff             @@
##           master   #19422      +/-   ##
==========================================
- Coverage   66.55%   66.39%   -0.17%     
==========================================
  Files        1670     1670              
  Lines       63824    63824              
  Branches     6510     6510              
==========================================
- Hits        42481    42374     -107     
- Misses      19654    19761     +107     
  Partials     1689     1689              

Flag	Coverage Δ
hive	?
mysql	81.86% <0.00%> (ø)
postgres	81.91% <0.00%> (ø)
presto	?
python	82.00% <0.00%> (-0.35%)	⬇️
sqlite	81.67% <0.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
superset/db_engine_specs/__init__.py	88.46% <0.00%> (ø)
superset/db_engines/hive.py	0.00% <0.00%> (-85.19%)	⬇️
superset/db_engine_specs/hive.py	70.00% <0.00%> (-15.77%)	⬇️
superset/db_engine_specs/presto.py	83.68% <0.00%> (-5.44%)	⬇️
superset/connectors/sqla/models.py	88.43% <0.00%> (-1.22%)	⬇️
superset/db_engine_specs/base.py	87.97% <0.00%> (-0.34%)	⬇️
superset/models/core.py	88.80% <0.00%> (-0.24%)	⬇️
superset/utils/core.py	89.71% <0.00%> (-0.12%)	⬇️

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[rusackas]

Wow, I'm really sorry this didn't get more attention earlier and get merged. We're thankful for your contribution nonetheless. If you ever submit more fixes that don't get attention in the future, please nag us here and on Slack. The squeaky wheel gets the grease!

In any case, it seems there was another PR that was merged, which fixes the same issue:
#20714

I'll close this one since that got in. Thanks again!