[Android] Raise NoDevicesError in DeviceUtils.parallel.

build/android/pylib/device/device_utils.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.
 
 """Provides a variety of device interactions based on adb.
 
 Eventually, this will be based on adb_wrapper.
 """
 # pylint: disable=unused-argument
 
@@ skipping 1410 matching lines @@
                from which DeviceUtils instances can be constructed. If None,
                all attached devices will be used.
       async: If true, returns a Parallelizer that runs operations
              asynchronously.
 
     Returns:
       A Parallelizer operating over |devices|.
     """
     if not devices:
       devices = adb_wrapper.AdbWrapper.GetDevices()
+      if not devices:
+        raise device_errors.NoDevicesError()
     devices = [d if isinstance(d, cls) else cls(d) for d in devices]
     if async:
       return parallelizer.Parallelizer(devices)
     else:
       return parallelizer.SyncParallelizer(devices)