Connection: 'level' is now 'source' for Errors

lib/Connection.js

@@ -942,7 +942,7 @@ Connection.prototype._resUntagged = function(info) {
     if (this.state === 'connected' && !this._curReq) {
       clearTimeout(this._tmrConn);
       var err = new Error('Received negative welcome: ' + info.text);
-      err.level = 'protocol';
+      err.source = 'protocol';
       this.emit('error', err);
       this._sock.end();
     }