Change ANONYMOUS_INITIALIZER from _WITHOUT_CONSTRUCTOR to _IN_TRAIT

"since we made all classes have constructors some time ago", this error
doesn't even trigger on a class such as:
class testClass { {} }
It does trigger on a trait:
trait testTrait { {} }

Therefore, the error message should be changed to reflect the new
behavior.
However, an actual quickfix for this will be more involved. It is not a
good idea to simply delete the initializer, since it could contain
valuable code that the user wants to keep.