hotplug: improve error message during group ownership change

hotplug: improve error message during group ownership change

procd currently outputs following error messages:

 procd: cannot set group dialout for /dev/ttyw8
 procd: cannot set group dialout for /dev/ttyq1
 procd: cannot set group dialout for /dev/ttywf

from which it's not clear where the problem is, if it's either getgrnam
or chown failing and why it's failing so this patch adds name of failed
function and its errno.

Ref: https://github.com/openwrt/openwrt/pull/1773#issuecomment-495555284
Signed-off-by: Petr Štetiar <ynezz@true.cz>