Simplify the error message when index.js couldn't be loaded (#22354)

In some cases, the loading failure of `index.js` is not related to the
ROOT_URL directly, ex: https://gitea.com/gitea/helm-chart/issues/392

If the user's reversed proxy is mis-configured:
`http://public-domain/gitea/xxx` -> `http://gitea:3000/gitea/xxx`, it
also causes the loading failure.

So this PR removes the ROOT_URL related tip from the error message.

templates/base/footer.tmpl

@@ -22,7 +22,7 @@
 		<script src='https://hcaptcha.com/1/api.js' async></script>
 	{{end}}
 {{end}}
-	<script src="{{AssetUrlPrefix}}/js/index.js?v={{AssetVersion}}" onerror="alert('Failed to load asset files from ' + this.src + ', please make sure the asset files can be accessed and the ROOT_URL setting in app.ini is correct.')"></script>
+	<script src="{{AssetUrlPrefix}}/js/index.js?v={{AssetVersion}}" onerror="alert('Failed to load asset files from ' + this.src + '. Please make sure the asset files can be accessed.')"></script>
 {{template "custom/footer" .}}
 </body>
 </html>