Similar to NameError#receiver, this returns the object on which
the modification was attempted. This is useful as it can pinpoint
exactly what is frozen. In many cases when a FrozenError is
raised, you cannot determine from the context which object is
frozen that you attempted to modify.
Users of the current rb_error_frozen C function will have to switch
to using rb_error_frozen_object or the new rb_frozen_error_raise
in order to set the receiver of the FrozenError.
To allow the receiver to be set from Ruby, support an optional
second argument to FrozenError#initialize.
Implements [Feature #15751]