It2MeHelperChannel error reporting cleanup

This CL introduces a new message type - Error. All error
message can only be reported in messages with this type.
This will provide a consistent error reporting mechanism with It2MeHelpeeChannel

Review URL: https://codereview.chromium.org/547013002

Cr-Commit-Position: refs/heads/master@{#293837}

remoting/webapp/background/it2me_helper_channel.js

@@ -119,7 +119,8 @@ remoting.It2MeHelperChannel.HangoutMessageTypes = {
   HELLO: 'hello',
   HELLO_RESPONSE: 'helloResponse',
   CONNECT: 'connect',
-  DISCONNECT: 'disconnect'
+  DISCONNECT: 'disconnect',
+  ERROR: 'error'
 };
 
 /** @enum {string} */
@@ -167,11 +168,7 @@ remoting.It2MeHelperChannel.prototype.onHangoutMessage_ = function(message) {
     throw new Error('Unknown message method=' + message.method);
   } catch(e) {
     var error = /** @type {Error} */ e;
-    console.error(error);
-    this.hangoutPort_.postMessage({
-      method: message.method + 'Response',
-      error: error.message
-    });
+    this.sendErrorResponse_(this.hangoutPort_, error, message);
   }
   return false;
 };
@@ -280,11 +277,7 @@ remoting.It2MeHelperChannel.prototype.onWebappMessage_ = function(message) {
     throw new Error('Unknown message method=' + message.method);
   } catch(e) {
     var error = /** @type {Error} */ e;
-    console.error(error);
-    this.webappPort_.postMessage({
-      method: message.method + 'Response',
-      error: error.message
-    });
+    this.sendErrorResponse_(this.webappPort_, error, message);
   }
   return false;
 };
@@ -309,3 +302,25 @@ remoting.It2MeHelperChannel.prototype.unhookPorts_ = function() {
     this.onDisconnectCallback_  = null;
   }
 };
+
+/**
+ * @param {chrome.runtime.Port} port
+ * @param {string|Error} error
+ * @param {?{method:string, data:Object.<string,*>}=} opt_incomingMessage
+ * @private
+ */
+remoting.It2MeHelperChannel.prototype.sendErrorResponse_ =
+    function(port, error, opt_incomingMessage) {
+  if (error instanceof Error) {
+    error = error.message;
+  }
+
+  console.error('Error responding to message method:' +
+                (opt_incomingMessage ? opt_incomingMessage.method : 'null') +
+                ' error:' + error);
+  port.postMessage({
+    method: remoting.It2MeHelperChannel.HangoutMessageTypes.ERROR,
+    message: error,
+    request: opt_incomingMessage
+  });
+};