Make nvd3 refresh smoother. (#1618)

superset/assets/javascripts/explore/explore.jsx

@@ -73,6 +73,7 @@ function query(forceUpdate, pushState) {
     // update the url after prepForm() fix the field ids
     history.pushState({}, document.title, slice.querystring());
   }
+  slice.container.html('');
   slice.render(force);
 }
 

superset/assets/javascripts/modules/superset.js

@@ -177,6 +177,9 @@ const px = function () {
         always(data);
         controller.error(this);
       },
+      clearError() {
+        $(selector + ' div.alert').remove();
+      },
       width() {
         return token.width();
       },