Fix error handling in create-profile

Fix #548

It's only a partial fix, and I haven't run it through the integration
tests, because they're not working.

/cc @drj11, note the callback fallthrough, I found your commit titled
"not very working error handling for signup", and I suspect this
was a large part to do with it. I was seeing multiple callbacks
to the useradd chain, which is pretty bad.

https://github.com/scraperwiki/custard/commit/52e023eedb66d79ca8caf639a5c747ba395366bf