Add custom error pages

app/templates/403.html.j2

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+{% extends "base.html.j2" %}
+
+{% block title %}Opaque - Forbidden{% endblock %}
+
+{% block page_content %}
+<div class="page-header">
+    <h1>Forbidden</h1>
+</div>
+{% endblock %}

app/templates/404.html.j2

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+{% extends "base.html.j2" %}
+
+{% block title %}Opaque - Page Not Found{% endblock %}
+
+{% block page_content %}
+<div class="page-header">
+    <h1>Not Found</h1>
+</div>
+{% endblock %}

app/templates/500.html.j2

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+{% extends "base.html.j2" %}
+
+{% block title %}Opaque - Internal Server Error{% endblock %}
+
+{% block page_content %}
+<div class="page-header">
+    <h1>Internal Server Error</h1>
+</div>
+{% endblock %}