Provide command in case fixtures haven't been ran

This commit adds the command necessary to generate fixtures in the
output of the error that asks if you haven't generated fixtures.

scripts/frontend/startup_css/get_startup_css.js

@@ -29,7 +29,9 @@ const mergePurgeCSSOptions = (...options) =>
 const getStartupCSS = async ({ htmlPaths, cssPaths, purgeOptions }) => {
   const content = htmlPaths.map((htmlPath) => {
     if (!fs.existsSync(htmlPath)) {
-      die(`Could not find fixture "${htmlPath}". Have you run the fixtures?`);
+      die(
+        `Could not find fixture "${htmlPath}". Have you run the fixtures? (bundle exec rspec spec/frontend/fixtures/startup_css.rb)`,
+      );
     }
 
     const rawHtml = fs.readFileSync(htmlPath);