remove the generic 400 error page handler

it just shows a h1, so isn't helpful for people. We can re-use the 500
error page, which includes instructings "Try again later" and
instructions on what to do next (check the status page, email notify
support).

this required refactoring to ensure we can show the 500 error page while
still returning the required status code

app/templates/error/400.html

@@ -1,11 +0,0 @@
-{% extends "withoutnav_template.html" %}
-{% block per_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 %}