Raise error if declared coverage not found

[lazyatom]

We can get the "build" to warn us and show the exception(s) if the expected coverage data is missing (or has a typo, more likely).

[h-lame]

Interesting - I have thought about just putting in all the {::coverage ...} tags so that folk raising PRs to add coverage just need to update the data and this'd break that possibility, but I can certainly see the appeal of this - we don't have much in the way of checks that this site actually does what we expect.

[h-lame]

Actually, I'm reminded of what a useful failure case might be; rather than complaining if coverage can't be found, what if we linted the json / yml files in data to make sure they're valid. Last month I asked a speaker to add a coverage link and their json had an extra comma in it, which I only spotted post-merge.