check main canonical broken

Providing a slightly more relevant error message. Perhaps this will show that Element is not properly aliases in this check.

```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}}
 ```