Fix incorrect handling of non-existent scopes

The old code seemed to assume that `scope` would be `None` if no
`Prefix` matching the network address could be found in the DB.  But,
of course, Django raises a `DoesNotExist` exception when using
`Prefix.objects.get()` for this purpose.

This changes the logic to catch the exception, but otherwise produce
the originally intended response.

python/nav/web/ipam/prefix_tree.py

@@ -467,10 +467,10 @@ def make_tree(prefixes, family=None, root_ip=None, show_all=None, sort_by="ip"):
     init = []
 
     if root_ip is not None and root_ip:
-        scope = Prefix.objects.get(net_address=root_ip)
-        if scope is not None:
+        try:
+            scope = Prefix.objects.get(net_address=root_ip)
             node = PrefixNode(scope)
-        else:
+        except Prefix.DoesNotExist:
             node = FauxNode(root_ip, "scope", "scope")
         init.append(node)