Return better error messages

[jonjohnsonjr]

This hoists the publisher creation out of the loop for ko publish to
avoid needlessly resolving credentials multiple times.

This pulls the publisher creation outside of resolveFilesToWriter so we
can fail earlier if e.g. KO_DOCKER_REPO is unset, otherwise we start up
the kubectl goroutine and it would asynchronously fail with:

error: no objects passed to apply

[jonjohnsonjr]

I'm thinking we could pull makePublisher and makeBuilder out into some initialization step at some point since they're dependency-injectiony and just parse options into these behaviors.