code: @Nonnull
      protected Throwable getResponseException(Throwable e) {
          // Unwrap execution exceptions.
          if (ExecutionException.class.isAssignableFrom(e.getClass())) {
              return getResponseException(e.getCause());
          }
          return e;
      }
docs: Get the exception to ge handled on failed requests.
  
  @param e The exception seen.
  @return The exception to use as response base.