Oddly enough, this template doesn't display error messages (of the sort handled by the messageStack class). I feel like I must be missing something, because surely in two years somebody would have noticed this?

The reason for the suppression of error messages is simple, it's the "display none" in this bit of the CSS:

.messageStackWarning, .messageStackError, .messageStackSuccess, .messageStackCaution {padding: .2em .5em;border: 1px solid #000;display:none}

For ZC to function correctly, that needs to be removed.