updatehub: report error before rollback on unconfirmed image

Report the error state to the UpdateHub server before triggering a rollback
in autohandler mode. This prevents the server from redeploying the same
(failed) update after rollback, avoiding update loops. Also exposes a new
public syscall: updatehub_report_error(), allowing manual mode users to
report an error explicitly.

Signed-off-by: Adrien Maillard <adrien.maillard@edu.hefr.ch>