[WIP] Tell users about incomplete challenge prerequisites.

This pull request tells users about incomplete challenge prerequisites, primarily the very first one that instructs them to add the main `<script>` tag. This particular one is very important, because without it, we can't actually detect if they've completed anything.

Note that there are a few wrinkles with this right now: if the user's code momentarily doesn't parse due to a very temporary syntax error (e.g. because the user is in the middle of typing something) then a prerequisite may register as being incomplete, which could derail the user's focus as their tutorial pane suddenly tells them that something they've already done isn't finished.