Fix: Exit endpoint if an error is occured in request or proxy

[alex-schneider]

This pull request fixes sending a defined request within an endpoint regardless if a possible error occurred with a proxy or/and request definition.

server "api" {
  endpoint "/" {
    proxy {
      url = "https://..."
    }
    # or/and
    request "sidekick" {
      url = "https://..."
    }

    response {
      status = 200
      body = "some content"
    }
  }
}

Now the response with status 200 will not be written and the internal server error with status 500 will be served if something goes wrong with proxy or request e.g. a connection or timeout error. This does not apply to origin responses which sends a status 500 and Couper had no issues while sending the request and reading the response.