Remove enum opkg_error.

Remove enum opkg_error.

The return codes aren't particularly useful and trying to have a useful one
for every error scenario would be like extending errno. Good error messages
via opkg_message() are far more appropriate.

git-svn-id: http://opkg.googlecode.com/svn/trunk@395 e8e0d7a0-c8d9-11dd-a880-a1081c7ac358