When multiple devices are attached, give possible device options.

BUG=

Review URL: https://codereview.chromium.org/138443003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@245252 0039d316-1c4b-4281-b951-d872f2087c98

tools/telemetry/telemetry/core/backends/chrome/android_browser_finder.py

@@ -177,8 +177,9 @@ def FindAllAvailableBrowsers(finder_options, logging=real_logging):
     return []
 
   if len(devices) > 1:
-    logging.warn('Multiple devices attached. ' +
-                 'Please specify a device explicitly.')
+    logging.warn(
+        'Multiple devices attached. Please specify one of the following:\n' +
+        '\n'.join(['  --device=%s' % d for d in devices]))
     return []
 
   device = devices[0]