Returns a 'failed' status when no covers are found

[dgellow]

If mpd is playing a song and the server can't find the cover /api/cover will returns the following json

{
  "status": "success",
  "large": null,
  "medium": null,
  "mega": null,
  "extralarge": null,
  "small": null
}

Something like that should be better

{
    "status" : "fail",
    "error" : "Can't find the cover",
    "cover" : null
}

[gcmalloc]

What if you only have a small size, what if the request between the flask server and last fm failed ?

[dgellow]

If not all size are found

{
  "status": "success",
  "error": null,
  "cover": {
      "large": "http//foo.bar.tld/album-picture.jpg",
      "medium": null,
      "mega": null,
      "extralarge": null,
      "small": null
  }
}

If the request between flask and lastfm failed

{
    "status" : "fail",
    "error" : "Can't fetch information from lastfm",
    "cover" : null
}