Spec::Matchers.create(:be_successful) do
  matches do |rack|
    rack.status == 200
  end

  message do |not_string, rack|
    "Expected #{describe_request(rack)}#{not_string} " \
    "to be successful, but it returned a #{rack.status}"
  end
end

def be_successful
  simple_matcher("response to be successful") do |response|
    response.status == 200
  end
end

or

def be_successful
  simple_matcher("response to be successful") do |response, matcher|
    matcher.failure_message = "expected successful response, got #{response.status}"
    matcher.negative_failure_message = "expected error response, got #{response.status}"
    response.status == 200
  end
end