treewide: Migrate to the new dream2nix API

packages/templates/src/error.html

@@ -0,0 +1,21 @@
+<extends src="./lib/html/base.html">
+  <block name="content">
+    <section class="section">
+      <div class="container">
+        <h1 class="title has-text-weight-normal">{{ error.status_code }}</h1>
+
+        <div class="columns">
+          <div class="column content">
+            <!-- prettier-ignore -->
+            <markdown>
+              {{ error.message }}
+
+              If you think this is a mistake, feel free to [contact
+              me](~/src/mail.html)!
+            </markdown>
+          </div>
+        </div>
+      </div>
+    </section>
+  </block>
+</extends>