def response
      return "Error" unless url
      uri = to_uri(url)
      uri ? response_of_uri(uri) : response_of_file(url)
    end
def response
      return "Error" unless url
      uri = to_uri(url)
      uri ? response_of_uri(uri) : response_of_file(url)
    end
Checks whether a link is reachable. If so code '200' is returned
 otherwise 'Error'
 #response expects that the including class has an attribute 'url'