viced: Allow GetCapabilities to return an error

At the moment, the GetCapabilities RPC handler ignors errors from
CallPreamble or CallPostamble. Instead of silently swallowing these,
and potentially reutnring a bogus capabilities structure, make
it possible for the RPC to return failure to the caller.

Caught by clang-analyzer

Reviewed-on: http://gerrit.openafs.org/9241
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
(cherry picked from commit ea0441520138d2129b5c6b2bd16c7640a8680146)

Change-Id: Ic3828198e43403506f991455f09924387b483879
Reviewed-on: http://gerrit.openafs.org/9534
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>