Merge: Better autoinit conflict message

On

~~~nit
class A
    var x: Int
end

class B
    var y: Bool
end

class C
    super A
    super B
end
~~~

The error message

> a.nit:9,7: Error: conflict for inherited inits a#B#init(y=) and a#A#init(x=)

now become

> a.nit:9,7: Error: cannot generate automatic init for class C. Conflict in the order in inherited initializers a#B#init(y=) and a#A#init(x=). Use `autoinit` to order initializers. eg `autoinit x=, y=`

(fell free to simplify or propose better)

Close #1549

Pull-Request: #1568
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>