rhn: Discard error message after displaying it

Previously the RHN page continuiously showed an error message even if it
was displayed already.
Now the message is dropped after it has been shown.

rhbz#903989

Change-Id: I4f3ab9e2bf7c2b78cfb975187fa8d61970538953
Signed-off-by: Fabian Deutsch <fabiand@fedoraproject.org>