Do not process FileNotFoundError in FileResponse, resolves #979

[toxadx]

FileNotFoundError can be processed in exception_handlers

Summary

Avoid handling FileNotFoundError in FileResponse.
This allows FileNotFoundError to be properly handled using exception_handlers, for example, by returning a 404 error when a file is not found.

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.