Revert "core: Fix error types."

This reverts commit 6df6e01ed5a581765d245bf7303cda4497d0eb22.

gpgme_error_t et al are from a time where we had no hard dependency on
libgpg-error. Now libgpg-error is a required and thus it does not
make sense to have these surrogates. In fact we should ventually
completely remove them.

• Signed-off-by: Werner Koch <wk@gnupg.org>