DAFS: Ensure GetVolume errors on ERROR volumes

DAFS: Ensure GetVolume errors on ERROR volumes

In GetVolume, after we call VAttachVolumeByVp_r, there is no explicit
check to see if vp is in VOL_STATE_ERROR state. Make sure we don't try
to use such a volume, or blindly transition the volume away from that
state.

Reviewed-on: http://gerrit.openafs.org/6165
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>
(cherry picked from commit f59312c0aee1a5376b29262efc6e6ea71264305a)

Change-Id: Ibdd5cb5c475409918cdad1e73e2d7ed4ef57bd13
Reviewed-on: http://gerrit.openafs.org/6321
Reviewed-by: Derrick Brashear <shadow@dementix.org>
Tested-by: Derrick Brashear <shadow@dementix.org>