Handle application-level/ipfs-based deletions

[Ullaakut]

Goal of this PR

Fixes #228
Fixes #230

• Makes the metadata fetcher return the status code of its requests
• Makes the addition worker look at the error field within its payload when it receives an Internal Server Error or Not Found Error from the fetcher
• Makes the addition stage use the sentinel error for ErrTokenNotFound to look for application-level/ipfs-based deleted tokens.

Draft

This PR remains a draft for now because it might have side effects from the fact that the lambda worker no longer fails for any status code that is different to 200. It now handles the case of 500 separately, and does not error if it's just a 500 that is not related to an app-level deletion, so should it still fail just with a different error? @awfm9

[awfm9]

Those are two different cases, they should be handled in separate clauses.

1. 500 Internal Server Error has a JSON error object with the TokenNot Found message;
2. 404 Not Found has a simply return string in the body, that contains the IPFS missing message.

[awfm9]

It should probably be something like:

if code == http.StatusInternalServerError && isTokenNotFound(body) {
  return nil, ErrTokenNotFound
}
if code == http.StatusNotFound && isIPFSMissingLink(body) {
  return nil, ErrTokenNotFound
}