Fix a Mercurial crash with 'hg outgoing' and only warnings.

When trying to post without any outgoing changes, and with warnings
(for, say, unverified HTTPS certificates), things would crash. It
assumed if there was any output, there would be at least one non-warning
line, but this isn't always safe. We now check for this.
Fixed a missing space in the "you are not authenticated" output.

There was a missing space between "Review Board" and "server," due to
how the line wrapped.
Remove unused import and variable declaration and add missing import

This is just some simple changes to make pyflakes happy (and fix git-p4
diffing).

Testing done: Unit tests pass.
Reviewed at https://reviews.reviewboard.org/r/4690/