integration fix: better error reporting in case of X2 failure

If the IP address for X2AP is not good, the error message reported
was very unclear. Attempt to make it better.

openair2/X2AP/x2ap_eNB.c

@@ -371,7 +371,10 @@ void x2ap_eNB_handle_sctp_init_msg_multi_cnf(
   /* Exit if CNF message reports failure.
    * Failure means multi_sd < 0.
    */
+  if (instance->multi_sd < 0) {
+    X2AP_ERROR("Error: be sure to properly configure X2 in your configuration file.\n");
     DevAssert(instance->multi_sd >= 0);
+  }
 
   /* Trying to connect to the provided list of eNB ip address */