fix: Update exception for DatasourceNotFound from DatasourceDao.get_datasource()

[hughhhh]

SUMMARY

Updating exception for DatasourceDAO.get_datasource to pass whenever a user tries to build a chart on a dataset that doesn't exist anymore.

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 #21217 (597a068) into master (0726840) will decrease coverage by 11.31%.
The diff coverage is 50.00%.

❗ Current head 597a068 differs from pull request most recent head 84dfa31. Consider uploading reports for the commit 84dfa31 to get more accurate results

@@             Coverage Diff             @@
##           master   #21217       +/-   ##
===========================================
- Coverage   66.26%   54.94%   -11.32%     
===========================================
  Files        1781     1781               
  Lines       67930    67931        +1     
  Branches     7255     7255               
===========================================
- Hits        45016    37327     -7689     
- Misses      21051    28741     +7690     
  Partials     1863     1863               

Flag	Coverage Δ
hive	53.17% <66.66%> (?)
mysql	?
postgres	?
presto	53.07% <66.66%> (+<0.01%)	⬆️
python	57.82% <66.66%> (-23.49%)	⬇️
sqlite	?
unit	50.73% <66.66%> (+<0.01%)	⬆️

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

Impacted Files	Coverage Δ
...gins/legacy-plugin-chart-world-map/src/WorldMap.js	0.00% <0.00%> (ø)
superset/explore/commands/get.py	31.03% <50.00%> (-58.63%)	⬇️
superset/dao/exceptions.py	100.00% <100.00%> (ø)
superset/utils/dashboard_import_export.py	0.00% <0.00%> (-100.00%)	⬇️
superset/key_value/commands/update.py	0.00% <0.00%> (-88.89%)	⬇️
superset/key_value/commands/delete.py	0.00% <0.00%> (-85.30%)	⬇️
superset/key_value/commands/delete_expired.py	0.00% <0.00%> (-80.77%)	⬇️
superset/dashboards/commands/importers/v0.py	15.62% <0.00%> (-76.25%)	⬇️
superset/datasets/commands/update.py	25.00% <0.00%> (-69.05%)	⬇️
superset/datasets/commands/create.py	29.41% <0.00%> (-68.63%)	⬇️
... and 283 more

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