Improve type error for invalid varargs argument

[kengorab]

Right now, varargs are typechecked as an Array, so the following code produces the following type error:

val arr = [1, 2, 3]
arr.push(4, 5, "6")

Type mismatch: (4:13)
  |  arr.push(4, 5, "6")
                 ^
Expected Int[], got String | Int[]

However, I'd expect the error that's raised to not have anything to do with arrays, since the user hasn't done anything with arrays. The error I'd expect to see is more like this:

Type mismatch: (4:13)
  |  arr.push(4, 5, "6")
                    ^^^
Expected Int, got String