Avoid logging an object on compilation errors

In certain configurations Storybook might log the whole Webpack
build object when warnings or errors are thrown. This was caused
by a `logger.error` call inside the `index.js` catch. This fixes by logging
the error only when it's an instance of an error.

app/react/src/server/index.js

@@ -157,4 +157,8 @@ Promise.all([webpackValid, serverListening])
       process.exit(0);
     }
   })
-  .catch(error => logger.error(error));
+  .catch(error => {
+    if (error instanceof Error) {
+      logger.error(error);
+    }
+  });