Fixup missing support for set_response_status within plain error_handler

[malud]

Previously a response block was required, now an existing AC error gets just the new status-code:

definitions {
  basic_auth "ba" {
    user = "hans"
    password = "peter"
    error_handler {
      set_response_status = 418
    }
  }
}

While accessing related endpoints without credentials this configuration will still throw a access control error: credentials required but changes the status-code from 401 to 418.

First implementation in #250