`E409` error from `npm publish` should not be relied upon as an "already published" signal

[mcous]

#232 added behavior to npm publish handling that treats an E409 error from npm as an unambiguous signal that the version in question has already been published. Upon investigation spurred by #244, this turns out not to be true. Multiple different cases can produce an E409

The E409 handling should be reverted so that it bubbles up to the caller, as other errors