tasks/cephfs: say which test failed in exception

Example:
Was: 'Test failure'
Now: Test failure: test_full_caps (tasks.mds_full.TestClusterFull)

Signed-off-by: John Spray <john.spray@redhat.com>
Merge pull request #170 from ceph/wip-cephfs-exception

tasks/cephfs: say which test failed in exception

Reviewed-by: Greg Farnum <greg@inktank.com>