Make UI more verbose if an error occurs

[kmille]

Right now, if the backend is down and you want to download something there is no error message shown to the user. The API responds with a 400 and a helpful message. So we can show it to the user. For example using youtube-dl:

1. enter: blubb
2. click download => nothing will happen
3. Backend responds with a 400:

{
  "error": "url is not a url. http... only"
}