web API should properly handle 404 errors

[alex-kovoy]

Currently, we server index.html for all unknown HTTP requests. We should change that to properly handle 404 errors which at the moment we do not.

[kontsevoy]

Lets do this:

Have a simple text-only error page which shows a generic 404 error but also prints the full URL which was not found. This is important for debugging misconfigured authentication callbacks.