DAFS: Do not transition to ERROR on trivial errors

DAFS: Do not transition to ERROR on trivial errors

attach2 can result in many different errors; some indicate that the
volume is in an inconsistent state, but many others just indicate that
the volume cannot be attached for benign reasons (such as VNOVOL if
the volume doesn't exist, or VOFFLINE if the volume is being used by a
volume utility). Currently, for DAFS, attach2 transitions the relevant
volume to the VOL_STATE_ERROR state for almost all errors encountered,
even the benign ones. Instead, skip the error state transition for
error handling paths that do not reflect a "broken" volume.

Change-Id: I07754f715920c4ee30fb156fb2412d1e1a7e2597
Reviewed-on: http://gerrit.openafs.org/6164
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>