feat: Add option to fail generating client if there are any warnings

[forest-benchling]

For our case, we'd rather fail fast than have a potentially incomplete client. I'm open to opinions on the best interface here, like if the flag should be renamed.

[forest-benchling]

@dbanty @emann (cc @dtkav @GitOnUp)

[codecov]

Codecov Report

Merging #427 (d1ad795) into main (032a4a4) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main      #427   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           47        47           
  Lines         1549      1548    -1     
=========================================
- Hits          1549      1548    -1     

Impacted Files	Coverage Δ
openapi_python_client/cli.py	100.00% <100.00%> (ø)
openapi_python_client/parser/openapi.py	100.00% <0.00%> (ø)
openapi_python_client/parser/properties/schemas.py	100.00% <0.00%> (ø)
...enapi_python_client/parser/properties/converter.py	100.00% <0.00%> (ø)

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[dbanty]

Excellent, thank you!