feat: customize error pages

see:
https://symfony.com/doc/current/controller/error_pages.html

webapp/templates/bundles/TwigBundle/Exception/error.html.twig

+{% extends 'app_base.html.twig' %}
+{% block title %} Error {{ status_code }} - {{ status_text }}{% endblock %}
+
+{% block body %}
+    <h1>Error {{ status_code }} {{ status_text }}</h1>
+
+    <div class="col-md-8 alert alert-danger" role="alert">
+        <h2>Oops, an error occurred</h2>
+        <p> The server returned a <em>"{{ status_code }} {{ status_text }}"</em>. </p>
+        <p> Something is broken. Please let us know what you were doing when
+            this error occurred. We will fix it as soon as possible.
+            Sorry for any inconvenience caused.
+        </p>
+    </div>
+    <p>
+        <a href="{{ path('app_home') }}">Return to the homepage</a>.
+    </p>
+
+{% endblock %}
+

webapp/templates/bundles/TwigBundle/Exception/error403.html.twig

+{% extends 'app_base.html.twig' %}
+{% block title %} Error {{ status_code }} - {{ status_text }}{% endblock %}
+
+{% block body %}
+    <h1>Error {{ status_code }} {{ status_text }}</h1>
+
+    <div class="col-md-8 alert alert-danger" role="alert">
+        <p> You are not authorized to display this page.  </p>
+    </div>
+    <p>
+        <a href="{{ path('app_home') }}">Return to the homepage</a>.
+    </p>
+
+{% endblock %}
+
+

webapp/templates/bundles/TwigBundle/Exception/error404.html.twig

+{% extends 'app_base.html.twig' %}
+{% block title %} Error 404 - Page not found {% endblock %}
+
+{% block body %}
+    <h1>Error 404</h1>
+
+    <div class="col-md-8 alert alert-danger" role="alert">
+        <h2>Page not found</h2>
+        <p> The requested page couldn't be located. </p>
+        <p> Checkout for any URL
+            misspelling or <a href="{{ path('app_home') }}">return to the homepage</a>.
+        </p>
+    </div>
+
+{% endblock %}