Add RenderMessage to new generateFormGet' (deprecation of non-prime)

From discussion: https://groups.google.com/forum/#!topic/yesodweb/lWKFzJDOnnY
I've discussed with Michael, it seems the right way to go.

Someone can check whether my warnings make sense?
I think it would be better without the WARNING actually, otherwise no one can use either version of the function with -Werror turned on.