Today S3 repositories ignore the failIfAlreadyExists parameter passed to the various write methods on BlobContainer because at the time of writing the S3 API had no way to implement this feature. However, S3 now supports overwrite protection. We should adopt this new functionality for S3 repositories. Moreover, all other repository implementations already support this feature, so with this change we can now strengthen the repository analysis API to verify that overwrite protection works as expected.