Improve the fifo open timeout error message in `android_shell.py`.

mojo/devtools/common/devtoolslib/android_shell.py

 # Copyright 2014 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 atexit
 import hashlib
 import logging
 import os
 import os.path
 import random
@@ skipping 108 matching lines @@
 
     def _run():
       def _wait_for_fifo():
         for _ in xrange(max_attempts):
           output = subprocess.check_output(fifo_command).strip()
           if output == fifo_path:
             return
           time.sleep(1)
         if on_fifo_closed:
           on_fifo_closed()
-        raise Exception('Unable to find fifo.')
+        raise Exception('Timed out waiting for shell the create the fifo file.')
       _wait_for_fifo()
       stdout_cat = subprocess.Popen(
           self._adb_command(['shell', 'run-as', _MOJO_SHELL_PACKAGE_NAME,
                              'cat', fifo_path]), stdout=pipe)
       atexit.register(_exit_if_needed, stdout_cat)
       stdout_cat.wait()
       if on_fifo_closed:
         on_fifo_closed()
 
     thread = threading.Thread(target=_run, name='StdoutRedirector')
@@ skipping 300 matching lines @@
           Results.output = rf.read()
 
     run_thread = threading.Thread(target=do_run)
     run_thread.start()
     run_thread.join(timeout)
 
     if run_thread.is_alive():
       self.stop_shell()
       return None, Results.output, True
     return None, Results.output, False