Handle the fact that the bugtracker uri is not in the correct format

Review Request #878 — Created May 26, 2009 and submitted — Latest diff uploaded

Review Board SVN (deprecated)
I specified the bugtracker uri of our internal bugzilla as http://www.mycompany.com/buzilla (also because the UI didn't have any hint about it)
This causes a 500 Error when trying to view a review, since the code assumes that the settings contain a valid format and it can simply do 

bug_url % bug_number

The patch at least handles the error in a bit more robust way instead of falling flat and logs a warning.

While the fix is needed for robustness, this should also be fixed at the ui level to prevent the error to start with.