Add support for an onError callback

Instead of cleaning the output folder on build start (like the `clean` option), I'd rather have it only clean it on build failure. I noticed there's a `onSuccess` handler, but it seems like an accompanying `onError` callback is missing. Any objection to adding this?

Start by locating the existing onSuccess handler and the build-start cleanup associated with the clean option. Trace the build failure path and define the callback behavior so the output folder is cleaned only after a failed build; done means an onError callback is available and invoked in that case.

Written by the indexing model from the issue text.