Errors and warnings are printed to stderr.

Fixes https://github.com/aws/aws-cli/issues/820

cc @jamesls @danielgtaylor 
Makes sense. It is an easy fix.  We just need to put an additional check in there that if ``self._num_parts == 0`` then do not print the progress bar probably somewhere around here:
https://github.com/aws/aws-cli/blob/develop/awscli/customizations/s3/executor.py#L312

I can make another pull request later to fix it.