sysupdated: Register known error types

sysupdated: Register known error types

This fixes a bug introduced during review of sysupdated. Originally,
we just returned EALREADY verbatim to signify that the target is
already up-to-date. Then we switched this to a proper error
(org.freedesktop.sysupdate1.NoCandidate) during review. But that now
maps to EIO, not EALREADY. Thus, whenever there's nothing to update,
updatectl would report I/O errors to the user, even though nothing
actually went wrong.