Make opr_uuid_toString return a status

Don't assume that converting a UUID to a string will always succeed.
Instead, opr_uuid_toString should return a status result to indicate
whether the operation was successful or not.

Change-Id: I49e6bf53b2a878342d3137510d2eca522e58604d
Reviewed-on: http://gerrit.openafs.org/9990
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>