Internal API: ConjectureData.freeze() can suppress exceptions

[jakkdl]

def test_frozen_things():
    # Hypothesis reraises the TypeError as a StopTest, because the data is Frozen,
    # and never resurfaces the TypeError anywhere.
    @given(st.data())
    def foo(data):
        data.conjecture_data.freeze()
        raise TypeError("oops")

    foo()

DataObject.conjecture_data is not public API (see #4170) so we shouldn't have to worry about users
directly calling data.conjecture_data.freeze(), but it might be possible that exceptions in internal code gets suppressed.