Refactorings

* Moved mappers to server module
* Dropped error generator from insight and @HttpStatusCode
* Always send error entity in case that client supports application/vnd.nexus-error-v1+xml or application/vnd.nexus-error-v1+json
* Always send validation errors entity in case that client supports application/vnd.nexus-validation-errors-v1+xml or application/vnd.nexus-validation-errors-v1+json
* Add infra for unit tests + unit tests for errors/validation errors