SessionNotFoundException thrown when getting not existing process instance

If you try to get not existing process instance using per process instance session strategy, you will get SessionNotFoundException. It should behave the same way regardless which session strategy is used and return null if the process instance no longer exists.

This has already been fixed for KIE Server in version 7 (see JBPM-5403) and the engine-level fix can be backported from there but additional changes in remote services will be needed as well.