attempt to deal with variable access to buckets

[petermao]

s3ed always errors out if s3:// is the only thing in the minibuffer, or if I don't have access to the bucket in question.

This is an attempt (though not pretty) to get around that limitation. If the current buffer is not an s3ed buffer, then instead of prepping the s3ed-completing-read with a bare s3://, I've inserted a completing-read to set current-s3-base-path from choices from s3ed-bucket-list.

Another way to deal with this is to change s3ed-completing-read to catch the "AccessDenied" errors.