Fixes #108. Ensures and exits cleanly when a download directory is not w...

...ritable.

BOOM!

<pre>
ruination:heirloom capen$ heirloom download -n liftoff-capen-chef-repo -i 'v1.0.0' -s secretkey -o /.fseventsd/
2013-02-14 11:47:53 -0800 ERROR : You don't have permissions to write to /.fseventsd/.
</pre>