Better error message for the check on main

Canonical realias is not working for main. This change demonstrates it more clearly.

```elm
Type Error:
Bad type for 'main'. It must have type Element or a (Signal Element)
Instead 'main' has type:

    Signal.Signal
      {element : Graphics.Element.ElementPrim,
       props : {color : Maybe.Maybe Color.Color,
                height : Int,
                hover : (),
                href : JavaScript.JSString,
                id : Int,
                opacity : Float,
                tag : JavaScript.JSString,
                width : Int}}
 
```