unzip: fix Mac build.

Turns out the Mac doesn't have <error.h>. Add our own "die" function
instead, and use it everywhere so the Mac isn't using an untested
codepath.

One upside to this is that we'll now call ourselves "unzip" even when
run as `ziptool unzip ...`, which was awkward to fix with <error.h> but
trivial if we're rolling our own anyway.

Test: still works on Linux
Change-Id: I9cb1922595a21cd9f6d55a70d67e30090f8b7f21