`onValidation` should communicate whether errors are visible or not

[dmnd]

trafficstars

Currently onValidation doesn't communicate whether errors are actually visible or not. Based on the feedback strategy, it's possible there are errors that the user cannot see. This is awkward.

Once this is fixed, we should call onValidation whenever validation changes but also whenever error visibility changes (which can happen onBlur)

[dmnd]

Probably a dupe of #26. Related to #27.