Errors and warnings are printed to stderr.

Fixes https://github.com/aws/aws-cli/issues/820

cc @jamesls @danielgtaylor 
Hmm, I just tested this and I cannot repo this output:
```
$ aws s3 mb s3://knapp-new-bucket-2
make_bucket: s3://knapp-new-bucket-2/

$ aws s3 sync . s3://knapp-new-bucket-2/
warning: Skipping file /Users/kyleknap/Documents/GitHub/aws-cli/tests/integration/customizations/s3/wierd_dir/unreadable. File/Directory is not readable.
upload: ./bar to s3://knapp-new-bucket-2/bar     
upload: ./foo to s3://knapp-new-bucket-2/foo
    
$ aws s3 sync . s3://knapp-new-bucket-2/
warning: Skipping file /Users/kyleknap/Documents/GitHub/aws-cli/tests/integration/customizations/s3/wierd_dir/unreadable. File/Directory is not readable.

$ 
```