agent: implement debugging support

lib/agent.js

@@ -13,7 +13,7 @@ import fetch from 'node-fetch';
  */
 
 export default class Agent {
-  constructor (host, { debug }) {
+  constructor (host, { debug } = {}) {
     this._host = host;
     this._debug = debug;
     this._initAgent();
@@ -28,7 +28,9 @@ export default class Agent {
 
   _onError (err) {
     // XXX: should we `this.emit()`?
-    if (this._debug) console.log('> [debug] connection error', err.stack);
+    if (this._debug) {
+      console.log('> [debug] agent connection error', err.stack);
+    }
     this._error = err;
   }