cause deploy to fail if `shift pull` fails on server

[nolessafool]

When, during a deploy, shift pull fails, the deploy continues and is marked successful at Netlify. We should improve the tooling so this doesn't happen.

cf https://app.netlify.com/sites/shift-docs/deploys/664bfe7b1e832500081e655c#L148-L152 which shows the problem.

[ionous]

fixed in #773