stack outputs a time stamp in error output

Perhaps this was in response to some earlier problem where we couldn't track down the time something happened, but it seems excessive to output a timestamp with all messages, e.g.

```
$ stack --verbose --stack /home/david/.config/stack/repos/github.com/bozemanpass/stack-test-stacks/stack-files/stacks/test-external-stack/ check
2026-04-03 01:02:49.863820: ERROR: stack name cannot be empty
```