viced: Improve CallPreamble error messages

These messages are not very useful right now. At least try to say what
host we sent an error to, so we know which host may be experiencing
some troubles as a result.

Change-Id: I8b41b46511ebd4760d5021ea2fe2011842450998
Reviewed-on: http://gerrit.openafs.org/9381
Reviewed-by: Mark Vitale <mvitale@sinenomine.net>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>