`Handler.init()` should return True/False

The declared API of `Handler.init()` is that it should return `True` if the handler was initialized successfully, or `False` if there was an issue and UI creation should be abandoned (in which case a `TraitError` is raised in `UI.prepare_ui()`).

However many examples, and a number of tests, have not returned anything (ie. so the return value is effectively `None`). The error-checking code has been testing for a return value of `False` rather than `if not handler.info(...)` so this code has been working fine.

As a result of this, there may be significant 3rd party code which returns `None` because it was following examples.

We should either:
- first warn, then raise an exception, when a non-bool value is returned from `Handler.init()`; or
- abandon the explicit return flag approach and simply expect that the `init` method simply raise an appropriate exception if there is an insurmountable issue with initializing the `Handler`.

See #1757 where the issue was first exposed and is partially addressed.