MDX parse errors cause the webpack compiler to hang

[gkaemmer]

If there is an error during MDX parsing, such as if you use <!-- markdown comment syntax --> or an unknown language with your code fences like ```foobar, the whole webpack process hangs (even after you fix the problem) and you have to stop and restart the dev server to get going again.

This looks to be because loader.ts exports an async function, which doesn't call callback(err) when it rejects.

I've got a fix coming up.