Make it possible to change the college a user is in.

While the general case is that we don't need this, and it's not
directly exposed at the moment, this is needed for user creation.

Change-Id: Ic7fbaf3e3401ab2dae00cb1f20ccba2aefc68f13
Add functionality to make a user either a student or a teacher.

This is needed for user account creation.

Change-Id: I356e99f8123ac480cd41c0b68196d126af5f9d0e
More useful exception details.

Include the specific user/college the error is about.

Change-Id: I04df6fb05ecf70a08a227fad2ab489cdd5ae4993