I developed this while going down a slightly different path. I tested it visually, but the case I tested doesn't exist. I'm confident it will work if we ever have an error on another select though.
Thankfully I was able to use basic DOM features, so there's no coupling of the logic with tom-select.
It wasn't going to be simple to get tom-select to listen for the 'changed' class on the original select, so I found a simple solution with a CSS sibling selector instead.
We should be able to use @extend .icon-chevron-down, but I couldn't get it to work. I'd like to have a better method for this, but we should upgrade our ancient FontAwesome before worrying about that.
