webadmin: Only close provider dialog if action succeeds

Give the dialog a chance to display an error message from the backend
before closing.

Change-Id: I8ecf9d1364935a2a9231b44131eca5ca725d6db6
Bug-Url: https://bugzilla.redhat.com/987949
Signed-off-by: Lior Vernia <lvernia@redhat.com>