add an option to return 400 and 500 reponse codes

[attachmentgenie]

I would like to be able to simulate a service that occasionally returns a error state instead of the expected response.

When providing this toxi as a percentage of all requests we can dial this up or down to see how the application responds to the responsive yet utterly broken downstream.

[Tcoop69]

Nice if I could do it.

[Tcoop69]

I don't understand it and I'm sad my heart is breaking . Wish I knew how to change my life fast.

[attachmentgenie]

closing in favor of #50