[android] Improve reverse forwarder error handling

build/android/pylib/forwarder.py

 # 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.
 
 import logging
 import os
 import re
 import sys
 import time
 
@@ skipping 70 matching lines @@
       raise Exception('Error while running adb forward.')
 
     (exit_code, output) = self._adb.GetShellCommandStatusAndOutput(
         '%s %s %s' % (tool.GetUtilWrapper(), Forwarder._DEVICE_FORWARDER_PATH,
                       Forwarder._DEVICE_ADB_CONTROL_PORT))
     if exit_code != 0:
       raise Exception(
           'Failed to start device forwarder:\n%s' % '\n'.join(output))
 
     for redirection_command in redirection_commands:
-      (exit_code, output) = cmd_helper.GetCmdStatusAndOutput(
-          [self._host_forwarder_path, redirection_command])
+      try:
+        (exit_code, output) = cmd_helper.GetCmdStatusAndOutput(
+            [self._host_forwarder_path, redirection_command])
+      except OSError as e:
+        if e.errno == 2:
+          raise Exception('Unable to start host forwarder. Make sure you have '
+                          'built host_forwarder.')
+        else: raise
       if exit_code != 0:
         raise Exception('%s exited with %d:\n%s' % (
             self._host_forwarder_path, exit_code, '\n'.join(output)))
       tokens = output.split(':')
       if len(tokens) != 2:
         raise Exception('Unexpected host forwarder output "%s", ' +
                         'expected "device_port:host_port"' % output)
       device_port = int(tokens[0])
       host_port = int(tokens[1])
       self._host_to_device_port_map[host_port] = device_port
@@ skipping 34 matching lines @@
 
   def DevicePortForHostPort(self, host_port):
     """Get the device port that corresponds to a given host port."""
     return self._host_to_device_port_map.get(host_port)
 
   def Close(self):
     """Terminate the forwarder process."""
     if self._device_process:
       self._device_process.close()
       self._device_process = None