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.
Is it that in cases where the repository and distributor do not exist are not returned as 404s? Do those cases return 400's because the URL is valid, but the non-URL parameters are not?