Improve logging in obscure cases when a client disconnects.

Closes #2017. Thanks to Craig Leres.

ChangeLog.txt

@@ -6,6 +6,7 @@ Broker:
 - Fix websockets listeners with TLS not responding. Closes #2020.
 - Add notes that libsystemd-dev or similar is needed if building with systemd
   support. Closes #2019.
+- Improve logging in obscure cases when a client disconnects. Closes #2017.
 
 Apps:
 - Allow command line arguments to override config file options in

src/loop.c

@@ -353,8 +353,11 @@ void do_disconnect(struct mosquitto *context, int reason)
 					case MOSQ_ERR_ADMINISTRATIVE_ACTION:
 						log__printf(NULL, MOSQ_LOG_NOTICE, "Client %s been disconnected by administrative action.", id);
 						break;
+					case MOSQ_ERR_ERRNO:
+						log__printf(NULL, MOSQ_LOG_NOTICE, "Client %s disconnected: %s.", id, strerror(errno));
+						break;
 					default:
-						log__printf(NULL, MOSQ_LOG_NOTICE, "Bad socket read/write on client %s, disconnecting.", id);
+						log__printf(NULL, MOSQ_LOG_NOTICE, "Bad socket read/write on client %s: %s.", id, mosquitto_strerror(reason));
 						break;
 				}
 			}else{