Check for S3 write permission and show warning if can't write

Resolves #124 

Checking the permissions of a user doesn't seem easy within the AWS SDK ([related](https://github.com/aws/aws-sdk-php/issues/121)), so went with this implementation.
Hmm, what if this fails? There's no error handling here is there?

You may want to actually put this stuff in its own function and run it before checking before checking the S3 permission and if it errors out, display an error message about not being able to write to disk.