[sql lab] improvements to the left panel (#2709)

* [sql lab] improvements to the left panel

* better error handling with error notifications
* table is added instantly with a loading image

* Fixing tests

* Fixed tests

superset/assets/javascripts/components/Loading.jsx

@@ -0,0 +1,27 @@
+import React from 'react';
+import PropTypes from 'prop-types';
+
+const propTypes = {
+  size: PropTypes.number,
+};
+const defaultProps = {
+  size: 25,
+};
+
+export default function Loading(props) {
+  return (
+    <img
+      className="loading"
+      alt="Loading..."
+      src="/static/assets/images/loading.gif"
+      style={{
+        width: props.size,
+        height: props.size,
+        padding: 0,
+        margin: 0,
+      }}
+    />
+  );
+}
+Loading.propTypes = propTypes;
+Loading.defaultProps = defaultProps;