Enhancement: Add warning for S3 if partial credentials are provided

Previously restic did not notify about incomplete credentials when using the
S3 backend, instead just reporting access denied.

Restic now checks that both the AWS key ID and secret environment variables are
set before connecting to the remote server, and reports an error if not.

https://github.com/restic/restic/issues/2388
https://github.com/restic/restic/pull/3532
