[Android] Return error exit code if no devices are attached.

build/android/pylib/base/test_dispatcher.py

 # Copyright 2013 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.
 
 """Dispatches tests, either sharding or replicating them.
 
 To dispatch, performs the following steps:
 * Create a test collection factory, using the given tests
   - If sharding: test collection factory returns the same shared test collection
     to all test runners
@@ skipping 342 matching lines @@
         unique test collection for each test runner.
     test_timeout: Watchdog timeout in seconds for running tests.
     setup_timeout: Watchdog timeout in seconds for creating and cleaning up
         test runners.
     num_retries: Number of retries for a test.
 
   Returns:
     A tuple of (base_test_result.TestRunResults object, exit code).
   """
   if not tests:
-    logging.error('No tests to run.')
+    logging.critical('No tests to run.')
     return (base_test_result.TestRunResults(), constants.ERROR_EXIT_CODE)
-
   if shard:
     # Generate a shared _TestCollection object for all test runners, so they
     # draw from a common pool of tests.
     shared_test_collection = _TestCollection([_Test(t) for t in tests])
     test_collection_factory = lambda: shared_test_collection
     tag_results_with_device = False
     log_string = 'sharded across devices'
   else:
     # Generate a unique _TestCollection object for each test runner, but use
     # the same set of tests.
     test_collection_factory = lambda: _TestCollection([_Test(t) for t in tests])
     tag_results_with_device = True
     log_string = 'replicated on each device'
 
   devices = _GetAttachedDevices(wait_for_debugger, test_device)
+  if not devices:
+    logging.critical('No attached devices.')
+    return (base_test_result.TestRunResults(), constants.ERROR_EXIT_CODE)
 
   logging.info('Will run %d tests (%s): %s', len(tests), log_string, str(tests))
   runners = _CreateRunners(runner_factory, devices, setup_timeout)
   try:
     return _RunAllTests(runners, test_collection_factory,
                         num_retries, test_timeout, tag_results_with_device)
   finally:
     try:
       _TearDownRunners(runners, setup_timeout)
     except android_commands.errors.DeviceUnresponsiveError as e:
       logging.warning('Device unresponsive during TearDown: [%s]', e)