Pass error messages from native messaging to web-app.

remoting/host/setup/host_starter.cc

Index: remoting/host/setup/host_starter.cc
diff --git a/remoting/host/setup/host_starter.cc b/remoting/host/setup/host_starter.cc
index 5eda05c52a6f1b285d2f06faaf22be1156227d1d..2fefc7cc552742cfcf2de9183067ffd2e6742bee 100644
--- a/remoting/host/setup/host_starter.cc
+++ b/remoting/host/setup/host_starter.cc
@@ -171,21 +171,30 @@ void HostStarter::StartHostProcess() {
   config->SetString("host_secret_hash", host_secret_hash);
   daemon_controller_->SetConfigAndStart(
       config.Pass(), consent_to_data_collection_,
-      base::Bind(&HostStarter::OnHostStarted, base::Unretained(this)));
+      base::Bind(&HostStarter::OnStartHostSucceeded, base::Unretained(this)),
+      base::Bind(&HostStarter::OnStartHostFailed, base::Unretained(this)));
 }
 
-void HostStarter::OnHostStarted(DaemonController::AsyncResult result) {
+void HostStarter::OnStartHostSucceeded() {
   if (!main_task_runner_->BelongsToCurrentThread()) {
     main_task_runner_->PostTask(FROM_HERE, base::Bind(
-        &HostStarter::OnHostStarted, weak_ptr_, result));
+        &HostStarter::OnStartHostSucceeded, weak_ptr_));
     return;
   }
-  if (result != DaemonController::RESULT_OK) {
-    unregistering_host_ = true;
-    service_client_->UnregisterHost(host_id_, access_token_, this);
+  base::ResetAndReturn(&on_done_).Run(START_COMPLETE);
+}
+
+void HostStarter::OnStartHostFailed(const std::string& error_message,
+                                    const tracked_objects::Location& location) {
+  if (!main_task_runner_->BelongsToCurrentThread()) {
+    main_task_runner_->PostTask(FROM_HERE, base::Bind(
+        &HostStarter::OnStartHostFailed, weak_ptr_, error_message, location));
     return;
   }
-  base::ResetAndReturn(&on_done_).Run(START_COMPLETE);
+
+  // start_host is a command-line tool, so the error has already been reported.
+  unregistering_host_ = true;
+  service_client_->UnregisterHost(host_id_, access_token_, this);
 }
 
 void HostStarter::OnOAuthError() {