Improve 404 error message (#10214)

options/locale/locale_en-US.ini

@@ -79,6 +79,8 @@ write = Write
 preview = Preview
 loading = Loading…
 
+error404 = The page you are trying to reach either <strong>does not exist</strong> or <strong>you are not authorized</strong> to view it.
+
 [startpage]
 app_desc = A painless, self-hosted Git service
 install = Easy to install

templates/status/404.tmpl

@@ -4,6 +4,7 @@
 	<p style="margin-top: 100px"><img src="{{StaticUrlPrefix}}/img/404.png" alt="404"/></p>
 	<div class="ui divider"></div>
 	<br>
+	<p>{{.i18n.Tr "error404" | Safe}}
 	{{if .ShowFooterVersion}}<p>Application Version: {{AppVer}}</p>{{end}}
 </div>
 {{template "base/footer" .}}