command_defaults: Don't crash on nonexistent command

It's common for ipa commands to raise NotFound in such a case.

Fixes: https://pagure.io/freeipa/issue/8954
Signed-off-by: Stanislav Levin <slev@altlinux.org>
Reviewed-By: Florence Blanc-Renaud <flo@redhat.com>

ipaserver/plugins/schema.py

@@ -292,6 +292,13 @@ class command_defaults(PKQuery):
     )
 
     def execute(self, name, **options):
+        if name not in self.api.Command:
+            raise errors.NotFound(
+                reason=_("{oname}: {command_name} not found").format(
+                    oname=self.name, command_name=name
+                )
+            )
+
         command = self.api.Command[name]
 
         params = options.get('params') or []