Make mandatory filter error message more verbose (#44139)

...by adding the undefined variable/attribute name, if available.

lib/ansible/plugins/filter/core.py

@@ -299,7 +299,11 @@ def mandatory(a):
 
     ''' Make a variable mandatory '''
     if isinstance(a, Undefined):
-        raise AnsibleFilterError('Mandatory variable not defined.')
+        if a._undefined_name is not None:
+            name = "'%s' " % to_text(a._undefined_name)
+        else:
+            name = ''
+        raise AnsibleFilterError("Mandatory variable %snot defined." % name)
     return a