Gracefully handle OSErrors in StaticFiles

[etr2460]

Summary

Instead of reraising the OSError, handle it and raise an HTTPException with a Bad Request status code response. Arguably this could be a 404 to avoid leaking anything, but a 400 seemed reasonable (and better than not catching the error).

Implements solution 1 to the discussion here: #2221

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.

[Kludex]

@etr2460 Thanks for the PR.

Given what I replied on #2221 (comment), I'll be closing this.