timepicker: make the component invalid if wrong input

[jnizet]

Using the demo with custom validation, if I enter something invalid (like aa, for example) in one of the fields, or if I clear one of the fields, I would expect a validation error other than "It's a bit too early", and the model to become null, since aa:<blank><blank> is not a valid input and should thus not, IMO, be saved as 00:00 in the model.

[pkozlowski-opensource]

Makes sense. Will look into those scenarios as part of alpha.2.