mgcp_client: mgcp_msg_gen: add more error logs

While adding CSD, this failed for me in add_lco without a descriptive
log message, so add more error messages.

Related: OS#4393
Change-Id: I4873a2db95525aab3e13046b645dd8f90e951466

src/libosmo-mgcp-client/mgcp_client.c

@@ -1392,8 +1392,10 @@ struct msgb *mgcp_msg_gen(struct mgcp_client *mgcp, struct mgcp_msg *mgcp_msg)
 	if (!use_sdp
 	    && (mgcp_msg->verb == MGCP_VERB_CRCX
 		|| mgcp_msg->verb == MGCP_VERB_MDCX)) {
-	        if (add_lco(msg, mgcp_msg) < 0)
+		if (add_lco(msg, mgcp_msg) < 0) {
+			LOGPMGW(mgcp, LOGL_ERROR, "Failed to add LCO, can not generate MGCP message\n");
 			return NULL;
+		}
 	}
 
 	/* Add mode */
@@ -1428,8 +1430,10 @@ struct msgb *mgcp_msg_gen(struct mgcp_client *mgcp, struct mgcp_msg *mgcp_msg)
 	if (use_sdp
 	    && (mgcp_msg->verb == MGCP_VERB_CRCX
 		|| mgcp_msg->verb == MGCP_VERB_MDCX)) {
-	        if (add_sdp(msg, mgcp_msg, mgcp) < 0)
+		if (add_sdp(msg, mgcp_msg, mgcp) < 0) {
+			LOGPMGW(mgcp, LOGL_ERROR, "Failed to add SDP, can not generate MGCP message\n");
 			return NULL;
+		}
 	}
 
 	if (rc != 0) {

tests/mgcp_client/mgcp_client_test.err

@@ -1,5 +1,6 @@
 DLMGCP MGW(mgw) MGCP client: using endpoint domain '@mgw'
 DLMGCP MGW(mgw) Message buffer to small, can not generate MGCP message (SDP)
+DLMGCP MGW(mgw) Failed to add SDP, can not generate MGCP message
 
 test_mgcp_client_cancel():
 DLMGCP MGW(mgw) MGCP client: using endpoint domain '@mgw'