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

Change-Id: Ic36e59e8cce8be4920f041a86d5636d54668d887
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>