Run: Don’t crash when the artifact configuration file is invalid

If you don’t set an artifact configuration file in your run.conf, the
artifact gets '/dev/null' as its configuration path.  You’d expect that
specifying '/dev/null' explicitly would work too, right?  Wrong!  Until
this commit, 'ppaml run' would try to save a copy of /dev/null, and
you’d get an exception when it realized that /dev/null was in a
different directory than run.conf.  This has now been corrected, and
you’ll get an error if you specify any kind of block device (actually,
anything that’s not a regular file or a directory) as an artifact
configuration path.
 Merge branch 'master' of git://github.com/GaloisInc/ppaml-client-tools.git