[NEXUS-6127] handle role and prov not found exceptions, as the model for the given user may invalid, or containing invalid entries
[NEXUS-6127] record priv/role lookup failures in response
record string not gstring for marshalling
Merge pull request #259 from sonatype/NEXUS-6127-handle-exceptions

TRIVIAL [NEXUS-6127] handle role and prov not found exceptions