BugWatch errors should be displayed on BugTracker pages

It would be nice if the list of bugwatches on a bugtracker page (e.g. http://launchpad.net/bugs/bugtrackers/sf) showed which bugwatches couldn't be updated due to errors. The method of displaying these errors on the bug pages themselves (where the bug watch icon is replaced by a warning triangle) could be used, with the alt text for the icon being the nature of the error.

This would allow us to see at a glance a rough proportion of erroring bug watches to correctly-updating ones.