With two exceptions, nice suggestions if feasible.

>>> def foo(*args=None): pass
SyntaxError: * argument cannot have default value

>>> def foo(**kwargs=None): pass
SyntaxError: ** argument cannot have default value

Good.

>>> foo(*args=[0])
SyntaxError: cannot assign to iterable argument unpacking

>>> foo(**args={"a": None})
SyntaxError: cannot assign to keyword argument unpacking

Incomprehensible.  It seems to me that these should have same message as first two; message should not depend on proposed default.