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.
Unfortunately the `bind` method is static, so the validation function must also be static. I agree it's not pretty, but I had a choice between copying and pasting code and doing static private(ish) methods. Since there are already several other static methods that are private(ish), I figured one more wouldn't make it any worse.