[Android] Raise NoDevicesError in DeviceUtils.parallel.

build/android/pylib/device/device_utils_test.py

 #!/usr/bin/env python
 # 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.
 
 """
 Unit tests for the contents of device_utils.py (mostly DeviceUtils).
 """
 
 # pylint: disable=C0321
@@ skipping 1386 matching lines @@
     with self.assertCall(
         mock.call.pylib.device.adb_wrapper.AdbWrapper.GetDevices(),
         [_AdbWrapperMock(serial) for serial in test_serials]):
       parallel_devices = device_utils.DeviceUtils.parallel()
     for serial, device in zip(test_serials, parallel_devices.pGet(None)):
       self.assertTrue(
           isinstance(device, device_utils.DeviceUtils)
           and serial == str(device),
           'Expected a DeviceUtils object with serial %s' % serial)
 
+  def testParallel_noDevices(self):
+    with self.assertCall(
+        mock.call.pylib.device.adb_wrapper.AdbWrapper.GetDevices(), []):
+      with self.assertRaises(device_errors.NoDevicesError):
+        device_utils.DeviceUtils.parallel()
+
 
 if __name__ == '__main__':
   logging.getLogger().setLevel(logging.DEBUG)
   unittest.main(verbosity=2)