make_subspack doesn't exit if passed unrecognized option

[gavinsdavies]

Running make_subspack with a typo in an option doesn't cause it to gracefully exit:

% make_subspack --with-padding /cvmfs/larsoft.opensciencegrid.org/spack-packages/ $PWD/emph-spack
getopt: unrecognized option '--with-padding'
Putting detail log in /tmp/make_subspack_19515.log
git clone-ing spack ....

I made a typo in the --with_padding option but if one doesn't pay close attention that it was not a valid option can be missed and it'll create the subspack minus the option one thought one has passed through.