WebImageFetcher: check better last.fm results

- make sure the XML with the album is the one we expect (<lfm> as
  root element)
- if the status is not "ok", inform whether the album was not found,
  or there was another error
- handle when the album has no cover

In all the above situations, WebImageFetcher does not attempt to start
a StoredTransferJob for an empty URL.