Improve error information

[eyurtsev]

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We're surfacing default error messages from Fast API / pydantic.

These messages are hard to understand; e.g., https://github.com/efriis/langchain-perplexaty/issues/2

We should investigate whether we can add an interceptor for exceptions to re-write the exceptions and make them easier to understand.

This should only affect endpoints that were added using add_routes without affecting any other endpoints