Prevent {{ url_for_node(...) }} crashing the planet when node doesn't exist.

Now None is returned as URL, and a warning is logged, rather than crashing
with a 500. A situation like this occurs when an activity refers to a
no longer existing node.

pillar/web/nodes/routes.py

@@ -593,7 +593,7 @@ def url_for_node(node_id=None, node=None):
             log.warning(
                 'url_for_node(node_id=%r, node=None): Unable to find node.',
                 node_id)
-            raise ValueError('Unable to find node %r' % node_id)
+            raise NotFound('Unable to find node %r' % node_id)
 
     return finders.find_url_for_node(node)