Add setup check to detect attempts to install on a path

Summary:
We don't support this and say so in the documentation, but can check explicitly.

https://github.com/facebook/phabricator/issues/148

Test Plan: Set base-uri to stuff with/without paths, verified setup caught mistakes and gave useful errors.

Reviewers: vrana, btrahan

Reviewed By: vrana

CC: aran

Differential Revision: https://secure.phabricator.com/D3768