Handling insert errors when posting to a form

I’m working on a ruby script to automate posting to a form. I have some
code like this:

  res=Net::HTTP.new(url.host, url.port).start {|http| http.request(req)
  p res
  case res
    when Net::HTTPSuccess, Net::HTTPRedirection
       puts "it worked"
       puts res.class
      puts "it didn't work"

The problem I have is that if the scripting langauge on the form (php in
this case) errors out, I don’t know since it will return a 200 response
code. Is there any way to access the error data better.