Added simple web page, routes are in a separate file

templates/500.html

@@ -0,0 +1,15 @@
+{% extends "base.html" %}
+
+{% block title %}500{% endblock %}
+
+{% block content %}
+    <h1>500: ISE</h1>
+    <p>
+        An error occured while
+        processing your request.
+    </p>
+    <p>
+        Please, try again later
+        or contact web site admin.
+    </p>
+{% endblock %}