[Webpublisher] Fix "Ready to stage" mechanism when the transition cannot be followed

This bug is related to the addon Webpublisher http://hg.nuxeo.org/sandbox/nuxeo-platform-webpublisher/

When marking documents ready to stage, the system recursively changes the lifecycle of the selected documents (and their children). It may happen that some documents are in a wrong state (deleted for example), then the operation fails because this document cannot follow the desired transition.

We have to check if the document can follow a transition.