Export does not bail on error

[chuntley]

Describe the bug
I'm exporting my sapper site and intentionally throwing errors in the pages. When export runs all of my pages return a 500 error. Instead of failing on error, the export process seems to continue processing all of the failing pages then report that the export was a success. It would make more sense to me to have export fail if an error is encountered to avoid "successfully" building a broken site.

Logs

Error: FAIL
    at getStuff (/Users/code/__sapper__/build/server/server.js:41:9)
    at processTicksAndRejections (internal/process/task_queues.js:93:5)
    at async get (/Users/code/__sapper__/build/server/server.js:247:23)
    at async handle_route (/Users/code/__sapper__/build/server/server.js:1266:5)
   2.45 kB   (500) page-1/index.html
> Finished in 7.0s. Type npx serve __sapper__/export to run the app.
✨  Done in 7.14s.

To Reproduce
Instructions in the bug description

Severity
Medium. My website builds on a cron and one of my data sources is flaky (for several hours at a time). I need to be able to skip that cron run when the website export failed.