qa/test_nfs: fix test failure when cluster does not exist
The patches that add object formatting / decorators to the nfs module
also made error handling more generic when accessing an nfs cluster and
now returns a nonzero exit code. A test was after the PR adding the
object format support that only checked an error message.
Update the test to match the new nfs module behavior as well as fixing a
typo.
Signed-off-by: John Mulligan <jmulligan@redhat.com>