update 404 page

404.md

@@ -7,7 +7,15 @@ permalink: /404.html
 
 <h4 class="text-body-secondary">404 error</h4>
 
-Oops! Something went wrong. Try [searching]({% link search.md %}) to find what you were looking for.
+Oops! Something went wrong. This page could not be found:
+
+<pre><code id="result"></code></pre>
+
+<script>
+document.getElementById("result").innerHTML = window.location.pathname;
+</script>
+
+Try [searching]({% link search.md %}) to find what you were looking for.
 You can [open an issue]({{ site.data.github.issue }}) or [contact me]({% link contact.md %}) to report a problem.
 
 {% include image.html