Return 404 instead of 500 for missing packages

[astrojuanlu]

The release of pip 10 broke the workflow for alternative PyPI servers, as it's explained in this issue by @popravich and which also affects pypi-server:

pypa/pip#5270

I haven't had time to properly debug the issue, but it seems that returning a nice 404 when the package is missing would fix the problem.

cc @AmitAronovitch @guydou

[astrojuanlu]

It seems that this project is not active anymore, so we're fixing this in our fork.

https://github.com/Juanlu001/pypi-server