Update error handling for prdcr_listen_add

Add error case when user attempts to create a producer listen that already exists.

ldms/src/ldmsd/ldmsd_request.c

@@ -9514,8 +9514,12 @@ static int prdcr_listen_add_handler(ldmsd_req_ctxt_t reqc)
 	pl = (ldmsd_prdcr_listen_t)
 		ldmsd_cfgobj_new_with_auth(name, LDMSD_CFGOBJ_PRDCR_LISTEN,
 						sizeof(*pl), NULL, 0, 0, 0);
-	if (!pl)
-		goto enomem;
+	if (!pl) {
+		if (errno == EEXIST)
+			goto eexist;
+		else
+			goto enomem;
+	}
 	pl->auto_start = 1;
 	if (disabled_start) {
 		if ((0 == strcmp(disabled_start, "1")) ||
@@ -9578,6 +9582,11 @@ static int prdcr_listen_add_handler(ldmsd_req_ctxt_t reqc)
 	(void)snprintf(reqc->line_buf, reqc->line_len,
 			"Memory allocation failed.");
 	goto send_reply;
+eexist:
+	reqc->errcode = EEXIST;
+	(void)snprintf(reqc->line_buf, reqc->line_len,
+			"The prdcr listener %s already exists.", name);
+	goto send_reply;
 einval:
 	reqc->errcode = EINVAL;
 	(void) snprintf(reqc->line_buf, reqc->line_len,