Bug 1048043 - Add message for when revision not found.

When treeherder doesn't know about a revision (either because it's not a legitimate revision or because that push just hasn't been processed yet), it just bails out completely, leaving a completely empty page aside from Treeherder's header UI. 

This adds a message for when no job info is found for the given revision, so it at least doesn't look like the page is just broken.