A missing manufacturer entry confuses responder moderation

[peternewman]

The admin homepage reports:
n Responders in the moderation queue

But when you click into it, it tells you there are no responders left to moderate.

This is because of:

rdm-app/templates/admin-moderate-responder.tmpl

Line 8 in 7d35c76

{% if manufacturer %}

rdm-app/templates/admin-moderate-responder.tmpl

Lines 143 to 153 in 7d35c76

	{% else %}
	{% if device_id %}
	Unknown manufacturer {{ manufacturer_id }}
	(0x{{ manufacturer_id|stringformat:"04x" }}). Check the
	<a href="http://tsp.esta.org/tsp/working_groups/CP/mfctrIDs.php" target="_blank">TSP</a> page.
	{% else %}
	No responders in the queue. <a href="/admin">Back</a> to the admin console or
	<a href="/admin?action=flush_cache">Flush the cache</a>.
	{% endif %}
	{% endif %}

Showing a more obvious error would be good at least, or ideally skipping onto the next fixture.