Add a helpful message if translation screenshot upload fails

Bug: 814906
Change-Id: I3707ab5bf8231ab0b8805bad5b645d3db9e85811
Reviewed-on: https://chromium-review.googlesource.com/1149062
Commit-Queue: Mustafa Emre Acer <meacer@chromium.org>
Reviewed-by: anthonyvd <anthonyvd@chromium.org>
Cr-Commit-Position: refs/heads/master@{#579521}

tools/translation/upload_screenshots.py

@@ -179,7 +179,8 @@ def main():
         num_threads=10,
         skip_hashing=False,
         gzip=None) != 0:
-      print 'Error uploading screenshots, exiting.'
+      print ('Error uploading screenshots. Try running '
+             '`download_from_google_storage --config`.')
       exit(1)
 
   print