Added a message that suggest to configure a proxy for a network.

chrome/browser/resources/chromeos/login/screen_error_message.css

 /* 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.
  */
 
 #error-message-screen-title {
   -webkit-margin-end: 60px;
   -webkit-margin-start: 60px;
   display: block;
   font-weight: bold;
   margin-bottom: 20px;
 }
 
 #error-message-body {
   -webkit-margin-end: 60px;
   -webkit-margin-start: 60px;
   display: block;
 }
 
+#captive-portal-proxy-message-text {
+  margin-top: 20px;
+}
+
 #captive-portal-network-select-text {
   margin-top: 20px;
 }
 
 .show-with-offline-message,
 .show-with-captive-portal,
 .show-with-proxy-error {
   display: none;
 }
 
 .show-offline-message .show-with-offline-message,
 .show-captive-portal .show-with-captive-portal,
 .show-proxy-error .show-with-proxy-error {
   display: block;
 }
 
+.show-captive-portal .dont-show-with-captive-portal {
+  display: none;
+}
+
 #error-guest-signin,
 #error-offline-login {
   -webkit-margin-start: 60px;
   margin-bottom: 21px;
 }