1116825 - Adding a non-existent user to a role now returns HTTP 400 instead of 404.

https://bugzilla.redhat.com/show_bug.cgi?id=1116825

`curl -X POST -d '{"login": "noone"}' -k -u admin:admin 'https://localhost/pulp/api/v2/roles/super-users/users/'` should result in 400.
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