Added a new 400.html error page, this error page will only be used when we get an HTTPError from the api.

This was done to solve the problem of invitations expiring and the user getting a 500 error message, which says to try again.

app/templates/error/400.html

@@ -0,0 +1,11 @@
+{% extends "withoutnav_template.html" %}
+{% block page_title %}Bad Request{% endblock %}
+{% block maincolumn_content %}
+<div class="grid-row">
+    <div class="column-two-thirds">
+      <h1 class="heading-medium">
+            {{ message|join(",")}}
+        </h1>
+    </div>
+</div>
+{% endblock %}