Convert add_remote to use GitReviewExceptions

By using the GitReviewException class and subclasses, the raised exceptions
will be caught in main() and properly handled. One Exception can be
converted to a CommandFailed exception, and the other is simple enough
to not need a subclass of GitReviewException.

Change-Id: Idb02d43fd480ab67493c757fb95431602239a55f
Merge "Convert add_remote to use GitReviewExceptions"