def get_digests(self, repo_name, tag):
if not hasattr(self, '<STR_LIT>'):<EOL><INDENT>bot.error('<STR_LIT>')<EOL>sys.exit(<NUM_LIT:1>)<EOL><DEDENT>return self.manifest['<STR_LIT>']<EOL>
return a list of layers from a manifest.
The function is intended to work with both version
1 and 2 of the schema. All layers (including redundant)
are returned. By default, we try version 2 first,
then fall back to version 1.

For version 1 manifests: extraction is reversed

Parameters
==========
manifest: the manifest to read_layers from