`super` outside of a class uses wrong syntax object for error

[Sam Phillips]

fun foo():
  super(1,2,3)

rhombus-prototype/rhombus/private/class-method.rkt:510:30: head: allowed only within methods and constructors in: head

https://github.com/racket/rhombus-prototype/blob/master/rhombus/private/class-method.rkt#L510