Show a pretty error when attempting to create a review from an empty changeset.

Review Request #681 — Created Dec. 27, 2008 and submitted — Latest diff uploaded


Review Board SVN (deprecated)


Fairly frequently (at least at VMware), people try to create a review request
from the web UI by entering a changeset number that has no associated open
files. In these cases, we get a KeyError for 'depotFile' when attempting to get
the change description from the Perforce Python API.
This change adds a check and an exception to show an error on the field instead
of just sending a traceback to my email and dropping them back on the form.
Ran scmtools unit tests.