cache: report errors when unlinking

Before:

$ abuild-apk del mutt
(1/3) Purging mutt (1.12.1-r0)
(2/3) Purging gdbm (1.18.1-r0)
(3/3) Purging libidn (1.35-r0)
OK: 3752 MiB in 805 packages
$ apk cache sync --purge || echo nope, sorry
$ ls /home/apk-cache/*gdbm*
/home/apk-cache/gdbm-1.18.1-r0.b0b17e93.apk

After:

$ apk cache sync --purge || echo nope, sorry
ERROR: Unable to delete gdbm-1.18.1-r0.b0b17e93.apk: Permission denied
nope, sorry