Refactor error handling in HTMLLoader, enhance error templates, and add new entry and setup templates

public/templates/entrys_frontend/entrys.html

@@ -0,0 +1,38 @@
+<!DOCTYPE html>
+<html lang="en">
+<head>
+  <link href="https://fonts.googleapis.com/css2?family=Roboto:wght@400;500;700&display=swap" rel="stylesheet">
+  <script type="importmap">
+    {
+      "imports": {
+        "@material/web/": "https://esm.run/@material/web/"
+      }
+    }
+  </script>
+  <script type="module">
+    import '@material/web/all.js';
+    import {styles as typescaleStyles} from '@material/web/typography/md-typescale-styles.js';
+
+    document.adoptedStyleSheets.push(typescaleStyles.styleSheet);
+  </script>
+
+  <style>
+    body {
+        display: flex;
+        justify-content: center;
+        align-items: center;
+        text-align: center;
+    }
+  </style>
+</head>
+<body>
+    <div class="content">
+        <h1 class="md-typescale-display-medium">Hello!</h1>
+        <p>Welcome to your Guestbook! Enter here your prefered username and password.</p>
+        <md-filled-text-field label="Username" type="text"></md-filled-text-field><br><br>
+        <md-filled-text-field label="Password" type="password"></md-filled-text-field><br><br>
+        <md-filled-text-field label="Repeat Password" type="password"></md-filled-text-field><br><br>
+        <md-filled-button disabled>Save</md-filled-button>
+    </div>
+</body>
+</html>