better s3_get_files message

[cole-brokamp]

s3_get_files first checks all file sizes and asks the user if it is okay to download all these files with a total file size; however, some of these files could already exist locally. Although the underlying use of s3_get does prevent the redownloading of these files, the initial message from s3_get_files still states that they will be downloaded.

We should refactor s3_get_files to check for all files first, then announce which files do and do not exist and request to download only those that do not exist.

[erikarasnick]

I meant to do this as a pull request. oops. Not sure if this is the best way to do this.. lots of repetition between s3_get and s3_get_files now.