Fix failure to serialize pydantic errors that are not directly json serializable

[dmontagu]

Applies jsonable_encoder to pydantic error output in case the context is not JSON serializable.

Closes #743

[codecov]

Codecov Report

Merging #748 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #748   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files         276    276           
  Lines        7201   7208    +7     
=====================================
+ Hits         7201   7208    +7

Impacted Files	Coverage Δ
fastapi/exception_handlers.py	100% <100%> (ø)	⬆️
tests/test_multi_body_errors.py	100% <100%> (ø)	⬆️

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[dmontagu]

@tiangolo something seems wrong with the CI -- my first submission of this PR had a failing test (and failed code coverage) and it still "passed all checks".

[tiangolo]

Thanks @dmontagu ! 🚀 🍰

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About the CI, I thought it was some temporal issue with Travis, I hadn't found any info about it yet. Because the builds still ran on Travis, although they weren't reported here.

But I just checked again, and it seems to be part of a migration from travis-ci.org to travis-ci.com: https://docs.travis-ci.com/user/migrate/open-source-on-travis-ci-com

I created this PR mainly to test it and it seems it's finally working correctly: #749