mender-artifact should exit with nonzero value after printing usage text

Currently, when mender-artifact prints usage text, for example when no command line parameters are supplied, it exits with status 0. A bug in a script could interpret this as a successful command (e.g. thinking it had validated a signed artifact), when it should instead notice that something is wrong. Mender-artifact should exit with a nonzero value in this case, or any case where the command line parameters are not correct.