If plugin returns a null result (Bad plugin), we transition into PLUGIN_FAILURE regarldess of whether or not it threw an exception.
PLUGIN_FAILURE does not mean call did not succeed at the gateway, it rather indicates that it is uncertain whether or not call has failed.
Merge branch 'master' of github.com:killbill/killbill
Make sure payment sytem thros an exception if payment plugin returns a null result
add sanity method to verify payment plugin result