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.

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>
(cherry picked from commit 1092cbe34fc8519826b3fa0565505b7bd81bc922)

Change-Id: I77e701378a999c8ad9413bfefcace0ee06aecb07
Reviewed-on: http://gerrit.openafs.org/9597
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Simon Wilkinson <simonxwilkinson@gmail.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>