Use a more meaningful message on AT send failure

src/at.c

@@ -94,7 +94,7 @@ gboolean at_send_command(struct EG25Manager *manager)
                     break;
 
                 default:
-                    g_warning("write(2): %s", strerror(errno));
+                    g_warning("error sending AT command: %s", strerror(errno));
                     at_next_command(manager);
                     return FALSE;
                 }