Handle missing command argument

When just running `run-one`, there's a strange error:

    flock: bad number: /home/user/.cache/run-one/68b329da9893e34099c7d8ad5cb9c940

The same applies to `keep-one-running`.

With `run-this-one` it is even more cryptic:

    lsof: WARNING: can't stat() fuse.gvfsd-fuse file system /root/.gvfs
          Output information may be incomplete.
    lsof: status error on /home/user/.cache/run-one/68b329da9893e34099c7d8ad5cb9c940: No such file or directory
    lsof 4.86
     latest revision: ftp://lsof.itap.purdue.edu/pub/tools/unix/lsof/
     latest FAQ: ftp://lsof.itap.purdue.edu/pub/tools/unix/lsof/FAQ
     latest man page: ftp://lsof.itap.purdue.edu/pub/tools/unix/lsof/lsof_man
     usage: [-?abhKlnNoOPRtUvVX] [+|-c c] [+|-d s] [+D D] [+|-f[gG]] [+|-e s]
     [-F [f]] [-g [s]] [-i [i]] [+|-L [l]] [+m [m]] [+|-M] [-o [o]] [-p s]
    [+|-r [t]] [-s [p:s]] [-S [t]] [-T [t]] [-u s] [+|-w] [-x [fl]] [--] [names]
    Use the ``-h'' option to get more help information.
    flock: bad number: /home/user/.cache/run-one/68b329da9893e34099c7d8ad5cb9c940

The programs should stop with an error and display usage information instead.