Catch database connection error and show error page

Closes: #5

templates/database_error.html

@@ -0,0 +1,17 @@
+{% extends "base.html" %}
+
+{% block title %}Geocode to Commons: database error{% endblock %}
+
+
+{% block content %}
+<div class="m-3">
+<h1>Geocode coordinates to Commons Category</h1>
+
+<div class="alert alert-danger" role="alert">
+  Error: database connection not available.
+</div>
+
+
+
+</div>
+{% endblock %}