I think that this shouldn't throw up through the stack - it should show an error page, the involved urls and a pointer to docs or some such.
screen to the user when (often) they can fix the situation themselves.
and when looking at ones own branch in launchpad).
Okay, so it sounds like the best fix would be for launchpad to have an analysis script, instead of having a custom exception handler in loggerhead to deal with this specific situation. Loggerhead itself normally actually shows the user the error it received--it's only on Launchpad that it hides the error. So I think a launchpad-specific resolution would be more appropriate here, and this probably does not need any work on loggerhead.
I disagree. Even for standalone loggerhead installs we can do better than barfing.