Webpack crash to console will keep sapper dev running in background

[naturalethic]

Repro:

1. sapper dev
2. Introduce a webpack error, for example I introduced a syntax error in a json file I was requiring
3. Sapper crashes to terminal with webpack error
4. ps ax | grep sapper -- it's still running
5. Run another sapper dev -- no port conflict announced
6. Boggle at why an older broken version of sapper is delivering you the wrong .sapper/client/<uid>/*.js files

[Rich-Harris]

I think this is a webpack bug — it's not calling the callback with an error like it's supposed to. I think the best workaround is to kill the child process if the main process dies — #182.

Luckily this seems to be limited to JSON parsing (and perhaps some other cases, but not JavaScript files or Svelte components)