chore: return 404 instead of 500 for datasource not found

[hughhhh]

SUMMARY

Errors bubbling up from DAO where the underlying datasource has been deleted, but then users who try to view a chart or dashboard are getting 500s instead of 404s. User's will see the following when they goto where the datasource is not found.

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 #22333 (efa701f) into master (2d30e9c) will decrease coverage by 1.47%.
The diff coverage is 40.00%.

@@            Coverage Diff             @@
##           master   #22333      +/-   ##
==========================================
- Coverage   66.86%   65.39%   -1.48%     
==========================================
  Files        1846     1847       +1     
  Lines       70510    70532      +22     
  Branches     7723     7729       +6     
==========================================
- Hits        47144    46121    -1023     
- Misses      21364    22408    +1044     
- Partials     2002     2003       +1     

Flag	Coverage Δ
hive	?
javascript	53.80% <ø> (+<0.01%)	⬆️
mysql	77.95% <40.00%> (-0.01%)	⬇️
postgres	78.02% <40.00%> (-0.01%)	⬇️
presto	?
python	78.14% <40.00%> (-3.10%)	⬇️
sqlite	76.48% <40.00%> (-0.01%)	⬇️
unit	?

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

Impacted Files	Coverage Δ
superset/charts/data/api.py	88.34% <40.00%> (-1.53%)	⬇️
superset/db_engines/hive.py	0.00% <0.00%> (-85.19%)	⬇️
...set/advanced_data_type/plugins/internet_address.py	16.32% <0.00%> (-79.60%)	⬇️
superset/utils/pandas_postprocessing/boxplot.py	20.51% <0.00%> (-79.49%)	⬇️
superset/charts/post_processing.py	11.76% <0.00%> (-77.95%)	⬇️
...perset/advanced_data_type/plugins/internet_port.py	18.75% <0.00%> (-77.09%)	⬇️
superset/utils/pandas_postprocessing/rolling.py	21.87% <0.00%> (-68.75%)	⬇️
...perset/utils/pandas_postprocessing/contribution.py	34.61% <0.00%> (-65.39%)	⬇️
superset/utils/pandas_postprocessing/resample.py	37.50% <0.00%> (-62.50%)	⬇️
superset/utils/pandas_postprocessing/select.py	38.46% <0.00%> (-61.54%)	⬇️
... and 64 more

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