Return 404 Not Found for accessing file as directory pattern

[xKerman]

fixes #1123

[xKerman]

Hmm, it seems that test suite failure on python 3.8 is caused by pytest update.
On my machine, test passes after downgrading pytest from 6.2.1 to 6.1.2.

[JayH5]

This seems like a good change, but my first thought is wondering if there are other exceptions we should be catching too, e.g. PermissionError 🤔 or even a more general error like OSError. I'm not really sure, and if somebody could do some experimentation/research that'd be great.