Added custom error pages

src/pretix/base/templates/500.html

@@ -0,0 +1,14 @@
+{% extends "error.html" %}
+{% load i18n %}
+{% block title %}{% trans "Internal Server Error" %}{% endblock %}
+{% block content %}
+    <i class="fa fa-bolt big-icon"></i>
+    <h1>{% trans "Internal Server Error" %}</h1>
+    <p>{% trans "We had trouble processing your request." %}</p>
+    <p>{% trans "If this problem persists, please contact us." %}</p>
+    <p>{{ exception }}</p>
+    <p>
+        <a href="javascript:history.back()">{% trans "Take a step back" %}</a>
+        &middot; <a href="javascript:location.reload()">{% trans "Try again" %}</a>
+    </p>
+{% endblock %}