Improve response when channel does not exist

[4www]

Currently it sends the plain text:

Error: could not handle the request

See https://api.radio4000.com/backup?slug=non-esisting-slug

We could define a error message object what we send, with at least a key message, and something explicit.

{
  "Code": 404,
  "Error": "Path not found",
  "ErrorDescription": "",
  "Details": {}
}

Example from: https://api.protonmail.ch/