lib: add helper method for 400 response with message

passbook/core/templates/error/400.html

@@ -19,6 +19,9 @@
     <h1>{% trans 'Bad Request' %}</h1>
 </header>
 <form>
+    {% if message %}
+    <p>{% trans message %}</p>
+    {% endif %}
     {% if 'back' in request.GET %}
     <a href="{% back %}" class="btn btn-primary btn-block btn-lg">{% trans 'Back' %}</a>
     {% endif %}