Ensured a generic template is used to render generic 404 errors, and a more specific one to render 'invalid short url' errors

module/Core/templates/invalid-short-code.phtml

@@ -0,0 +1,19 @@
+<?php $this->layout('ShlinkCore::layout/default') ?>
+
+<?php $this->start('title') ?>
+    <?= $this->translate('URL Not Found') ?>
+<?php $this->end() ?>
+
+<?php $this->start('stylesheets') ?>
+    <style>
+        p {margin-bottom: 20px;}
+        body {text-align: center;}
+    </style>
+<?php $this->end() ?>
+
+<?php $this->start('main') ?>
+    <h1><?= $this->translate('Oops!') ?></h1>
+    <hr>
+    <p><?= $this->translate('This short URL doesn\'t seem to be valid.') ?></p>
+    <p><?= $this->translate('Make sure you included all the characters, with no extra punctuation.') ?></p>
+<?php $this->end() ?>