vos: noise messages when attempting to delete non-existent volume

With vos refactor commit f4e73067cdef990262c69c38ac98761620a63f25,
some formerly conditional volume deletes are now unconditional.
This regresses 'vos move' output with harmless "error" messages
when AFSVolTransCreate() returns VNOVOL:
  "Failed to start transaction on <volume>"
  "Volume not attached, does not exist, or not online"

Modify DoVolDelete() to return early (and silently) with VNOVOL
in this case, allowing the caller to handle this appropriately.

Change-Id: I21a72e38f330335adf97ac50a7ac7d0fb97c1a9c
Reviewed-on: http://gerrit.openafs.org/9596
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>