catch user page exceptions and provide more information to the user

We need to handle this case better:

A user clicks on a @username which does not exist on the service which they are subscribed to. Doing so gives a blank tab for that @username. We should instead say "This user does not exist on $SERVICE."

Being smart and trying to guess where the user actually is beyond the scope of this bug.

Catching an exception in get_user_messages() (in all service.py that are effected) is probably the place to add this functionality. get_data() in microblog/__init__.py throws a URLError exception which we can work with.