Make ServeDir return 404 if file does not exists

[pbzweihander]

Description

Make ServeDir return 404 Not Found response when the requested file does not exist.

Motivation and Context

ServeDir, the internal static file serving endpoint, returns 500 when the file does not exist. But I expected 404.

How Has This Been Tested?

ServeDir did not have any test, so I added some unit tests that call the call method directly and some integration tests that request to ServeDir using Route::serve_dir.

Unfortunately, I could not find any test case to produce a 401 Forbidden error. The http_types::Url type seems to normalize the path segments so I couldn't produce any URL path that refers outside of the desired directory.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[yoshuawuyts]

@pbzweihander Thanks so much for this PR; these changes are great! -- a few nits aside I think we can merge this!