Add a better error message when using --revision-range on Perforce repository

Review Request #2614 — Created Sept. 24, 2011 and submitted — Latest diff uploaded




When using post-review in a Perforce repository, if users included the --revision-range parameter, post-review would spin out with a confusing error message.

This patch displays something a little more useful, including a link to Review Board's documentation on Perforce's range path syntax.
Added a quick test to ensure that using --revision-range on a Perforce repository causes us to exit out.