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
diff --git a/src/libosmo-mgcp-client/mgcp_client.c b/src/libosmo-mgcp-client/mgcp_client.c
index c3ecc9f..0434e70 100644
--- a/src/libosmo-mgcp-client/mgcp_client.c
+++ b/src/libosmo-mgcp-client/mgcp_client.c
@@ -1392,8 +1392,10 @@
 	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 @@
 	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) {