Merge pull request #745 from eduardoflorence/statemixin-error-message

Provide error for StateMixin obx widget

@@ -122,13 +122,15 @@ abstract class GetNotifier<T> extends Value<T> with GetLifeCycleBase {
 }
 
 extension StateExt<T> on StateMixin<T> {
-  Widget obx(NotifierBuilder<T> widget, {Widget onError, Widget onLoading}) {
+  Widget obx(NotifierBuilder<T> widget, {Widget Function(String error) onError, Widget onLoading}) {
     assert(widget != null);
     return SimpleBuilder(builder: (_) {
       if (status.isLoading) {
         return onLoading ?? CircularProgressIndicator();
       } else if (status.isError) {
-        return onError ?? Text('A error occured: ${status.errorMessage}');
+        return onError != null
+            ? onError(status.errorMessage)
+            : Text('A error occured: ${status.errorMessage}');
       } else {
         return widget(value);
       }