Remove 'except Exception' guard from `TraitType.clone`

[From #1645]

Currently, in most cases `TraitType.clone` with a new default value tries to validate that default value using `self.validate(None, None, default_value)`. This can fail for trait types like `This` which need a proper object and name for correct validation, so this code executes inside a `try / except Exception` wrapper.

Those trait types should override `clone` to implement their own behaviour, and then we can remove the catch-all-exceptions logic.