awesome-rebuild: exit non-zero in case of install failure
awesome-rebuild: exit non-zero in case of install failure

`makepkg -i` will exit with 0 in case the pacman DB is locked etc [1].
Returning non-zero makes it possible to not restart awesome in that case
etc.

1: https://lists.archlinux.org/pipermail/pacman-dev/2016-March/021003.html

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