added a errors template

templates/error/no_cards_in_collection.html

@@ -0,0 +1,6 @@
+{% extends 'base.html' %}
+
+{% block content %}
+    <h1>Error: No cards foud</h1>
+    <p>No cards were found in your collection. Consider <a href="{{ url_for('upload.index') }}">uploading</a> some.</p>
+{% endblock %}