Improve integration with JAX-RS async and reactive client API

[jpraet]

The current problem integration with JAX-RS async client API is incomplete, as we didn't have a concrete use case for it yet at CBSS.

• When Future.get throws ExecutionException(ProblemWrapper(Problem)) we unwrap to Problem.
  Perhaps it should be unwrapped to ExecutionException(Problem) as users of the Future API expect it to throw ExecutionException if it fails?
• Add missing support for https://docs.oracle.com/javaee/7/api/javax/ws/rs/client/InvocationCallback.html#failed-java.lang.Throwable-
• Add missing support for the reactive client API
• ...?