tasks: handle failure cleanly in test_stale_caps

Previously would fail because the cap waiter
completed too soon, without noticing that the
reason it completed quickly was because it failed.

Signed-off-by: John Spray <john.spray@redhat.com>
tasks: typo in mds_client_recovery

Signed-off-by: John Spray <john.spray@redhat.com>
tasks: fix race in test_stale_caps

Signed-off-by: John Spray <john.spray@redhat.com>