campaignd: Log errno to stderr if deleting an add-on archive failed

Unhandled remove() return value issue found by coverity.

This is about the most we can do about this here since internal server
errors like this shouldn't matter to clients (remove() failing is only
an issue if write_config() somehow fails or the same add-on is
reuploaded later).

Note to self: sanitize campaignd logging later so we can actually
distinguish between informational and error messages without including
the word "error" or "failure" and variations thereof in each line.