Feat/improve serde error handling

[elpiel]

• Introduce additional details in Addons' ResourceResponse when deserialization fails

• Introduce serde_path_to_error which, if addon returns a badly formatted JSON, will result in a clear error where the mistake is located
  NB: On items inside each of the different response types we still skip any item which is not deserializable (e.g. missing a mandatory field)

• Refactor APIResult to use tagged enum variant (related to API change https://github.com/Stremio/stremio-api/pull/184, error is no longer set if it's null)
  Given a bad response of {"unknown_variant": {"test": 1}}
  When APIResult does not match any variant it returns a vary vague error:
  data did not match any variant of untagged enum APIResult

with this change we now return a much cleaner and well understood error:
unknown variant `unknown_variant`, expected `error` or `result` at line 1 column 18