use computed.bool instead of directly messages length

because `computed.or` does not coalesce to bool, we otherwise end up with the number of errors instead of true in `isInvalid` and `isInvalidAndTouched`

addon/components/paper-input.js

@@ -38,7 +38,10 @@ export default Component.extend(FocusableMixin, ColorMixin, ChildMixin, Validati
 
   iconComponent: 'paper-icon',
 
-  isInvalid: computed.or('validationErrorMessages.length', 'isNativeInvalid'),
+  hasErrorMessages: computed.bool('validationErrorMessages.length'),
+
+  isInvalid: computed.or('hasErrorMessages', 'isNativeInvalid'),
+
   hasValue: computed('value', 'isNativeInvalid', function() {
     let value = this.get('value');
     let isNativeInvalid = this.get('isNativeInvalid');