Support exists checks with DreamObjects Swift API

Swift URLs with container and blob name take these forms:

    https://region-a.geo-1.objects.hpcloudsvc.com/v1/account/container/blob
    https://objects.dreamhost.com/swift/v1/container/blob

ParseSwiftErrorFromHttpResponse is too specific in this case; instead
interpret 404 errors directly via FalseOn404.