feat(messages): support for showError

showError is a shortcut for
options.validation.errorExistsAndShouldBeVisible

Author: Kamil Kisiela

src/wrappers/messages/messages.html

@@ -1,7 +1,7 @@
 <formly-transclude></formly-transclude>
-<div ng-messages="fc.$error">
+<div ng-messages="fc.$error" ng-show="showError">
     <div ng-repeat="(name, message) in ::options.validation.messages"
          ng-message-exp="name">
         {{message(fc.$viewValue, fc.$modelValue, this)}}
     </div>
-</div>
+</div>