Fix broken powershell_script resource

The powershell commands were not being guarded by our code that
checks for for exit codes

The regression happened at https://github.com/opscode/chef/commit/1b3705ca383b64092c9ef54d7e46201fab6271c7#diff-59352d07d2bfdded1139103819565cfaL38

cc @opscode/client-engineers