libadmin: Catch VL_GetNewVolumeId failures

If ubik_VL_GetVolumeId fails in the vos ProcessEntries routine,
report the failure and give up, rather than continuing with a
0 maxVolid

Caught by coverity (#988417)

Change-Id: I281e1c518813f9d7fab46e4f87a091fac6b3a754
Reviewed-on: http://gerrit.openafs.org/9268
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>