webamdin: clarify error message on storage creation

Clarified error message in the webadmin when trying
to create a file storage domain with a used connection:
"Create operation failed. Storage connection is already
used by the following storage domain: {storage_name}."

Change-Id: Ifc91bc60e42c94b972bb8c2c26a2980072bfe43e
Bug-Url: https://bugzilla.redhat.com/1029934
Signed-off-by: Daniel Erez <derez@redhat.com>