Ticket 587 - Replication error messages in the DS error logs
Bug Description: Replicand DNA error message are seen as vague.
Fix Description: Moved some DNA plugin errors messages to log as SLAPI_LOG_PLUGIN,
instead of SLAPI_LOG_FATAL. Made some replication errors
more human readable. Also, improved NSPR error str function
to check for zero result and not report "unknown" error.
Reviewed by: nhosoi(Thanks Noriko!)