improve error message on invalid @:op function signature (closes #2787)
improve error message on invalid @:op function signature (closes #2787)

pleclech/hacking-haxe,pleclech/hacking-haxe,pleclech/hacking-haxe,pleclech/hacking-haxe,pleclech/hacking-haxe,pleclech/hacking-haxe