This reverts part of
151f03378f s3: fix upload of single files into buckets without create permission
This erroneously assumed that a HEAD request on a non existent object
would return "NotFound" if the bucket was found. In fact it returns
"NotFound" when the bucket isn't found also.
This will break the fix for #4297 - however that can be made to work
using the new --s3-assume-bucket-exists flag
801a820c54