http - fixed runtime index out of range

Hi,

I fell over this issue while replicating the angularjs tutorial in gopherjs.
(You can find my fork of the tut [here](https://github.com/cryptix/GopherAngularTutorial) if you are interested but I'm not done yet)

This gives you a nicer error message when you don't supply enough arguments in your callback.
So far you only get `runtime error: index out of range` which doesn't help much.

I also thought about turning `RequestCallback` into something like `func(interface{}, int)` instead of just `interface{}` but i'm to new to gopherjs and it would break other usages of the callback I think.