fix: Ensure DatabaseErrorMessage works when extra is undefined

[john-bodley]

SUMMARY

At Airbnb we have custom error types and thus SupersetError.extra field may be undefined (or underdefined) per here whereas the DatabaseErrorMessage expects it to be well defined. This resulted in the application crashing when a custom error was thrown.

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

TESTING INSTRUCTIONS

ADDITIONAL INFORMATION

• Has associated issue:
• 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

[john-bodley]

Clean up.

[codecov]

Codecov Report

Merging #15848 (83850a9) into master (e660de6) will decrease coverage by 0.15%.
The diff coverage is 66.66%.

❗ Current head 83850a9 differs from pull request most recent head b561a23. Consider uploading reports for the commit b561a23 to get more accurate results

@@            Coverage Diff             @@
##           master   #15848      +/-   ##
==========================================
- Coverage   77.04%   76.89%   -0.16%     
==========================================
  Files         984      984              
  Lines       51787    51793       +6     
  Branches     7028     7028              
==========================================
- Hits        39901    39827      -74     
- Misses      11661    11741      +80     
  Partials      225      225              

Flag	Coverage Δ
hive	?
mysql	81.53% <ø> (ø)
postgres	81.56% <ø> (+<0.01%)	⬆️
python	81.64% <ø> (-0.29%)	⬇️
sqlite	81.19% <ø> (ø)

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

Impacted Files	Coverage Δ
...frontend/src/dashboard/components/Header/index.jsx	66.42% <0.00%> (-0.97%)	⬇️
superset/exceptions.py	93.18% <ø> (ø)
superset/views/base.py	75.86% <ø> (ø)
...rc/explore/components/controls/TextAreaControl.jsx	82.35% <83.33%> (-0.99%)	⬇️
...c/components/ErrorMessage/DatabaseErrorMessage.tsx	94.73% <100.00%> (ø)
superset/db_engines/hive.py	0.00% <0.00%> (-82.15%)	⬇️
superset/db_engine_specs/hive.py	69.44% <0.00%> (-17.07%)	⬇️
superset/views/database/mixins.py	81.03% <0.00%> (-1.73%)	⬇️
superset/db_engine_specs/presto.py	83.36% <0.00%> (-1.06%)	⬇️
superset/db_engine_specs/base.py	88.28% <0.00%> (-0.40%)	⬇️
... and 3 more

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[etr2460]

lgtm, thanks for the fix