Failing to publish should fail the task

[pimterry]

Currently failure at the publish step (getting to publishItem but getting an error response from the webstore) doesn't actually fail the task. I think it pretty unambiguously should. The issue is here: https://github.com/c301/grunt-webstore-upload/blob/master/tasks/webstore_upload.js#L344. This needs to inform Grunt of the failure.

Should be an easy fix, but I'm a bit busy atm. Might get time to take a look at fixing this myself maybe in future, but not sure, so just filing it here for now.