Simplify form error input handling

The form-input class now has the directive responsible for setting
the error classes. It watches its scope and sets the errors only
if the model is defined. This guard against undefined keeps the
error class from being set until validation passes, which is the
case when local validation succeeds, but in a digest cycle after the
server returns a failure notification this will trigger the error
class.
    
The form-validate directive is now just a controller forform-input to
register a callback to force error checking on submit.
I'd like us to stop using `"C"` in favour of always using an attribute. Using the class names like this mixes styling with behaviour. If we always use attributes it's very easy to see where directives have been applied to elements when viewing the markup.