Add 500.html template, missed in previous commit

templates/errors/500.html

@@ -0,0 +1,9 @@
+{%extends "base/page.html"%}
+{%block title%}Server error{%endblock%}
+{%block contents%}
+<h1>Server Error</h1>
+<p>
+{{message}}
+</p>
+</h1>
+{%endblock%}