Improve error messaging around failed deployments (particularly of functions)

Currently when customers do something "interesting" with functions (there's a large class of things that count as interesting; see related issue: https://github.com/netlify/bitballoon/issues/1628), we give a very non-useful error message while failing the deploy:

10:37:50 AM: Failing build: Failed to deploy site
10:37:50 AM: failed during stage 'deploying site': Failed to execute deploy: Upload cancelled: submission-created

This comes from https://github.com/netlify/buildbot/blob/d1d46200977e9388ad296974ec8dacb61a1c47bc/bot/stage_deploy.go#L50

@calavera asserts that it should be improved here:

https://github.com/netlify/open-api/blob/master/go/porcelain/deploy.go#L389

if sharedErr.err != nil {
  return fmt.Errorf("Upload cancelled: %s -- %v", f.Name, sharedErr.err)

And maybe once that messaging is improved, buildbot can be changed to surface the reason rather than just that there was an inscrutable error.

Read go/porcelain/deploy.go around the sharedErr handling near line 389, then inspect buildbot/bot/stage_deploy.go around line 50. Reproduce or trace the failed deployment path described in the issue and determine how the underlying function-deployment reason is currently lost. Done means the deploy error exposes a useful failure reason instead of only "Upload cancelled," with buildbot surfacing it where applicable.