Ticket #388 - Improve replication agreement status messages
    
    Bug Description:  Result codes that were negative values triggered a generic error
                      message (System Error).  This is because of mozLDAP's ldap_err2string()
                      which can only handle positive values, but openldap's ldap_err2string
                      can handle both positive and negative numbers.
    
    Fix Description:  Created a wrapper function (slapi_err2string), that can handle both
                      positive and negative error codes, regardless which ldap library is
                      being used.
    
    Reviewed by:
    
    https://fedorahosted.org/389/ticket/388
    (cherry picked from commit 8f21ac81cfe8bf70b555ae011c9e7ff953deeb8e)