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.

src/libsystemd/sd-bus/bus-common-errors.c

@@ -153,5 +153,7 @@ BUS_ERROR_MAP_ELF_REGISTER const sd_bus_error_map bus_common_errors[] = {
         SD_BUS_ERROR_MAP(BUS_ERROR_REBALANCE_NOT_NEEDED,         EALREADY),
         SD_BUS_ERROR_MAP(BUS_ERROR_HOME_NOT_REFERENCED,          EBADR),
 
+        SD_BUS_ERROR_MAP(BUS_ERROR_NO_UPDATE_CANDIDATE,          EALREADY),
+
         SD_BUS_ERROR_MAP_END
 };

src/libsystemd/sd-bus/bus-common-errors.h

@@ -158,4 +158,6 @@
 #define BUS_ERROR_REBALANCE_NOT_NEEDED         "org.freedesktop.home1.RebalanceNotNeeded"
 #define BUS_ERROR_HOME_NOT_REFERENCED          "org.freedesktop.home1.HomeNotReferenced"
 
+#define BUS_ERROR_NO_UPDATE_CANDIDATE          "org.freedesktop.sysupdate1.NoCandidate"
+
 BUS_ERROR_MAP_ELF_USE(bus_common_errors);