====== problem ======
  * https://github.com/zalando/problem
  * REST API에서의 Exception 과 Error 를 JSON 으로 자연스럽게 처리하는 기법
  * [[springframework:problem_spring_web|Problem Spring Web]]
  * [[https://www.baeldung.com/problem-spring-web|A Guide to the Problem Spring Web Library | Baeldung]]
  * proposes a common approach for expressing errors in REST API implementations
  * compatible with ''application/problem+json''