Fix FrozenInstanceError displaying empty message in tracebacks

[bysiber]

Summary

FrozenInstanceError (and FrozenAttributeError) display an empty error message in tracebacks despite having a msg class attribute.

@attr.s(frozen=True)
class Frozen:
    x = attr.ib()

Frozen(1).x = 2
# attr.exceptions.FrozenInstanceError        <-- no message!
# Expected: attr.exceptions.FrozenInstanceError: can't set attribute

The root cause is that FrozenError defines args as a class variable but never calls super().__init__() with the message. CPython's BaseException.__str__() reads from the C-level args slot which stays empty since __init__ was never passed any arguments.

The fix adds __init__ that passes self.msg to super().__init__(), so str(e) now correctly returns "can't set attribute".