Add msg parameter to the mandatory filter (#56724)

* Add msg parameter to the mandatory filter

The `mandatory` filter would be more useful, particularly when dealing with nested dictionaries, with the simple addition of a `msg` parameter for supplying it with a custom failure message.

lib/ansible/plugins/filter/core.py

@@ -280,7 +280,7 @@ def to_uuid(string):
     return str(uuid.uuid5(UUID_NAMESPACE_ANSIBLE, str(string)))
 
 
-def mandatory(a):
+def mandatory(a, msg=None):
     from jinja2.runtime import Undefined
 
     ''' Make a variable mandatory '''
@@ -289,7 +289,12 @@ def mandatory(a):
             name = "'%s' " % to_text(a._undefined_name)
         else:
             name = ''
-        raise AnsibleFilterError("Mandatory variable %s not defined." % name)
+
+        if msg is not None:
+            raise AnsibleFilterError(to_native(msg))
+        else:
+            raise AnsibleFilterError("Mandatory variable %s not defined." % name)
+
     return a