Only list system description with manifest files

Instead of just checking for directories in the .machinery path the
existence of the manifest.json file is verified.
This does prevent directories without manifest.json being listed with
`machinery list`.
Don't show json parsing errors in `list`

Only show a placeholder error message in `machinery list` for
descriptions which couldn't be parsed.
Also explain how to see the full error message. As soon as the
validate command is implemented the placeholder message should
be updated.