Ticket 48197 - error texts from preop plugins not sent to client

Bug Description: SLAPI_PLUGIN_BE_PRE_MODIFY_FN failed operations do not
                 retrieve the error text in ldbm_modify.c

Fix Description: Get the error text when we receive an error.

https://fedorahosted.org/389/ticket/48197

Reviewed by: lkrispenz(Thanks!)

ldap/servers/slapd/back-ldbm/ldbm_modify.c

@@ -645,6 +645,7 @@ ldbm_back_modify( Slapi_PBlock *pb )
 					opreturn = SLAPI_PLUGIN_FAILURE;
 					slapi_pblock_set(pb, SLAPI_PLUGIN_OPRETURN, &opreturn);
 				}
+				slapi_pblock_get(pb, SLAPI_PB_RESULT_TEXT, &ldap_result_message);
 				goto error_return;
 			}
 			/* The Plugin may have messed about with some of the PBlock parameters... ie. mods */