Add friendly crash page

templates/500.html

@@ -0,0 +1,20 @@
+{% extends "base.html" %}
+{% block title %}500 · Something Went Wrong · myboker.org{% endblock %}
+{% block page_class %}page--error{% endblock %}
+{% block content %}
+
+<div class="error-pg error-pg--500">
+    <div class="error-pg__suit" aria-hidden="true">♦</div>
+    <div class="error-pg__body">
+        <span class="error-pg__code">500</span>
+        <h1 class="error-pg__title">Something went wrong.</h1>
+        <p class="error-pg__sub">The app hit an unexpected error. Try again in a minute, or contact support if it keeps happening.</p>
+        <p class="error-pg__trace">Incident {{ crash_id }}</p>
+        <div class="error-pg__actions">
+            <a class="btn btn--primary btn--lg" href="{{ url_for('public.home') }}">Go home →</a>
+            <a class="btn btn--ghost btn--lg" href="{{ url_for('public.help') }}#contact">Contact support</a>
+        </div>
+    </div>
+</div>
+
+{% endblock %}