Errors and warnings are printed to stderr.

Fixes https://github.com/aws/aws-cli/issues/820

cc @jamesls @danielgtaylor 
It depends on the order of the files.  For example:
```
$ ls
bar baz foo
$ aws s3 sync . s3://jamesls-test-temp/
warning: Skipping file /private/tmp/syncme/baz. File/Directory is not readable.
Completed 0 part(s) with ... file(s) remaining


$ sudo mv baz unreadable
$ ls
bar        foo        unreadable
$ aws s3 sync . s3://jamesls-test-temp/
warning: Skipping file /private/tmp/syncme/unreadable. File/Directory is not readable.

$
```