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.
    
    https://fedorahosted.org/389/ticket/587
    
    Reviewed by: nhosoi(Thanks Noriko!)