If some classes couldn't be converted, add an error comment to notify.

If one or more of the classes couldn't be converted, there should be an automatic error comment to notify, underneath the converted classes:

```css
height: 2.5rem; 
color: #ffffff; 

/*
Can't convert:
border-t-[1px]
border-t-[var(--border-color)]
*/
```

Instead of silently leaving it out.