Parent issue: https://github.com/torrust/torrust-tracker/issues/141 - Keep the current API behaviour: URLs and response codes. - Write tests for all endpoints and all cases for each endpoint. - Refactor to use one struct for each error (rejection). See this [PR](https://github.com/torrust/torrust-tracker/pull/133/files).
