vlserver: Warn when we cannot unhash deleted entry

vlserver: Warn when we cannot unhash deleted entry

If we are trying to delete an entry from the vldb, we fail with
VL_NOENT if we cannot find the given entry on one of its hash chains.
This is indicative of corruption in the vldb (since we have an entry
not on a hash chain), but we don't really indicate this clearly. There
are no log messages, and the user running 'vos' only sees an error
like this:

    $ vos delentry 123456
    Could not delete entry for volume 123456
    You must specify a RW volume name or ID (the entire VLDB entry will be deleted)
    VLDB: no such entry
    Deleted 0 VLDB entries

Which is the exact same error message if the user tries to delete a
volume that does not actually exist.

We currently do not have an error code that clearly says that the
database appears corrupted and needs to be fixed, but we can at least
log an error in VLLog for this case, to give the administrator a
chance at fixing the situation. So, log a message in this situation.

Change-Id: I4f0ee8749a90441e1f8d779890293dc5d1d9dbee
Reviewed-on: https://gerrit.openafs.org/13382
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>