main: Fix exit code on grant/revoke command error

Client command grant/revoke was returning success exit
code on some failures (site not configured, arbitrator,
no tickets given). Test case is running
booth grant -s $IP; echo $?, where IP is not configured in the config
file.

Patch fixes this behavior so error code is returned.

Signed-off-by: Jan Friesse <jfriesse@redhat.com>