Fix unit test runner to exit with error when no devices are attached.

BUG=238474

Review URL: https://chromiumcodereview.appspot.com/14960003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@198914 0039d316-1c4b-4281-b951-d872f2087c98

build/android/pylib/gtest/dispatch.py

@@ -131,8 +131,7 @@ def _RunATestSuite(options, suite_name):
     attached_devices = android_commands.GetAttachedDevices()
 
   if not attached_devices:
-    logging.critical('A device must be attached and online.')
-    return 1
+    raise Exception('A device must be attached and online.')
 
   # Reset the test port allocation. It's important to do it before starting
   # to dispatch any tests.