Fix POST response status codes

[RobertLucian]

Closes #1208.

It appears that with the FastAPI web framework, throwing exceptions in the middleware is not the intended way of handling errors. It is assumed that any exceptions occurring in the middleware are gracefully handled, meaning that instead of throwing exceptions, responses are expected to be returned instead. It is therefore assumed that any exceptions occurring in the middleware are just implementation errors and thus returning the 500 status code is acceptable (aka what we've been returning so far).

The only place where the @app.exception_handler handlers are used is right after the middleware once an API route has already been taken (e.g. predict or summary). In order for those handlers to work, an API route handler would have to throw an exception that would match one of the handlers' exceptions.

A couple of resources that helped me with this are the following:

• Raise exception in python-fastApi middleware fastapi/fastapi#1304
• https://fastapi.tiangolo.com/tutorial/middleware/
• Exceptions raised in middleware handled by differently fastapi/fastapi#458
• https://fastapi.tiangolo.com/tutorial/handling-errors/

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checklist:

• run make test and make lint
• test manually (i.e. build/push all images, restart operator, and re-deploy APIs)
• update docs and add any new files to summary.md (view in gitbook after merging)

[vishalbollu]

Your predictor can return different types of objects such as JSON-parseable, string, and bytes objects. Navigate to the API responses section to learn about how to configure your predictor to respond with different response codes and content-types.