Return an error exit status from client commands that fail

Commands like `wemux pair` should exit with a non-zero status when the command
fails.  In my specific use case, .bash_profile attempts to attach to wemux, if
running, and falls back to a different command if not running.

    $ wemux pair
    No wemux server to pair with on 'wemux'.
    $ echo $?
    126

I believe this patch covers all the client commands. I haven't looked at the
server commands, so you may want to do that own your own.