Sysupgrade process shouldn't continue if the firmware image couldn't be
downloaded.
Ref: http://lists.infradead.org/pipermail/openwrt-devel/2019-December/020940.html
Reported-by: Petr Novák <petrn@me.com>
Signed-off-by: Petr Štetiar <ynezz@true.cz>
(cherry picked from commit cf3da66d2ce11a30ae2993b56276ade10c9bddb9)