lib: BaseRepoController: use webob.exc.HTTPNotFound if possible

In the entire code base, the use of 'paste' is very limited.
In particular, 'paste.httpexceptions' is only still used in lib/base.py, in
three occurrences:
- two of them in class BasicAuth which derives from paste.auth.something.
Here it probably makes sense to use paste.httpexceptions.
- one in BaseRepoController, which has no specific relation to paste. This
occurrence can be replaced with webob.exc like in the rest of the code
base.