Make the extract filter return Undefined on KeyError

groups['x']|map('extract', hostvars, 'somevar') would break if any host
didn't have 'somevar' set. With this change, it will return Undefined
instead. This change permits |map('extract', …)|map('default', 42) to
set a default value in such cases.

lib/ansible/plugins/filter/core.py

@@ -363,7 +363,10 @@ def extract(item, container, morekeys=None):
         if not isinstance(morekeys, list):
             morekeys = [morekeys]
 
+        try:
             value = reduce(lambda d, k: d[k], morekeys, value)
+        except KeyError:
+            value = Undefined()
 
     return value