Show more different NaCl loading errors.

ppapi/native_client/src/trusted/plugin/sel_ldr_launcher_chrome.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "native_client/src/include/nacl_macros.h"
 #include "ppapi/native_client/src/trusted/plugin/nacl_entry_points.h"
 #include "ppapi/native_client/src/trusted/plugin/sel_ldr_launcher_chrome.h"
 
+#include "ppapi/cpp/var.h"
+
 LaunchNaClProcessFunc launch_nacl_process = NULL;
 
 namespace plugin {
 
 bool SelLdrLauncherChrome::Start(const char* url) {
   NACL_NOTREACHED();
   return false;
 }
 
 bool SelLdrLauncherChrome::Start(PP_Instance instance,
                                  const char* url,
                                  bool uses_irt,
                                  bool uses_ppapi,
                                  bool enable_ppapi_dev,
                                  bool enable_dyncode_syscalls,
-                                 bool enable_exception_handling) {
+                                 bool enable_exception_handling,
+                                 nacl::string* error_message) {
+  *error_message = "";
   if (!launch_nacl_process)
     return false;
+  PP_Var var_error_message;
   // send a synchronous message to the browser process
   if (launch_nacl_process(instance,
                           url,
                           PP_FromBool(uses_irt),
                           PP_FromBool(uses_ppapi),
                           PP_FromBool(enable_ppapi_dev),
                           PP_FromBool(enable_dyncode_syscalls),
                           PP_FromBool(enable_exception_handling),
-                          &channel_) != PP_NACL_OK) {
+                          &channel_,
+                          &var_error_message) != PP_NACL_OK) {
+    pp::Var var_error_message_cpp(pp::PASS_REF, var_error_message);
+    if (var_error_message_cpp.is_string()) {
+      *error_message = var_error_message_cpp.AsString();
+    }
     return false;
   }
   return true;
 }
 
 }  // namespace plugin