1022553 - pulp-admin unbind commands will now return a user-friendly error message if the consumer or repository given don't exist.

https://bugzilla.redhat.com/show_bug.cgi?id=1022553

I've added validation to make sure the resources exist, and the proper exceptions are now raised if they don't. Because I was in the area, I fixed the documentation regarding which 4XX code will be returned when binding and unbinding.

This validation forced me to mock the distributor query manager in a number of tests which weren't setting up a repository when binding.
A MissingResource ultimately results in a 404 being returned, while an InvalidValue results in a 400 being returned. The consumer ID is part of the URL and everything else is a parameter, so this results in a 404 when the consumer id doesn't exist, and a 400 if one of the parameters is invalid. 

This is obviously pretty confusing without reading through the exceptions doc blocks and I'm not sure it's the "right" way to do it.