Don't say "update package missing" unless we mean it.

unzip_to_file reports failures itself these days, so there's it's unhelpful
of the caller to just guess what might have gone wrong.

Bug: http://b/21558406
Change-Id: I1e3d06c6cf902b8c6ef333dc60fd8f49680a493b
(cherry picked from commit acdbe92c)