form.field.Text: dynamically adding errors

This feature is important for server-side validations. E.g. a form where field values are getting checked VS BE-rules.

This feature feels like an epic, since it requires multiple changes.

1. isValid() needs to check if there is an errorText (or error) in place.
2. changing errorText needs to update the UI.
3. we need a new validate method which checks for client side errors (on value change)