On error, exit status code should be different than zero (0)

[iperdomo]

When running su-exec with wrong parameters, you get the Usage: [...] text, but the exit status code is zero (0) which hides the problem on systems relying on status codes signals, e.g. CI environments or shell scripts using set -e

$ docker run --rm -ti alpine:3.7 sh
/ # apk add --no-cache su-exec=0.2-r0
fetch http://dl-cdn.alpinelinux.org/alpine/v3.7/main/x86_64/APKINDEX.tar.gz
fetch http://dl-cdn.alpinelinux.org/alpine/v3.7/community/x86_64/APKINDEX.tar.gz
(1/1) Installing su-exec (0.2-r0)
Executing busybox-1.27.2-r7.trigger
OK: 4 MiB in 12 packages
/ # su-exec xxxx
Usage: su-exec user-spec command [args]
/ # echo $?
0