Permission errors should be more descriptive

[Jorge Bernal]

If I go to https://launchpad.net/sprints/ubz/+attend I got an error:

Sorry, you don't have permission to access this page.

You are logged in as Jorge Bernal.

This is too generic. If I'm not allowed to do that, I'd like to know why.
A message like "You can't register your attendance because you are not a member of ubzrs-team" would be more appropiate.

[Dafydd Harries (daf)]

Hmm, this would require some changes to the permissions system. I think if we could attach a message to an action protected by a permission so that that message gets displayed when the permission is not met (excuse my terminology abuse), then that would address the problem.

[Steve Alexander (stevea)]

There is some security refactoring coming up (oddly enough for some speed optimisation) that will allow us to say what teams or roles you need to be in / have to do that thing. This will help explain why a particular action or page is not allowed.