Improve error messaging when missing components in flow definition during cluster copy

I recently diagnosed an issue where a user copied a flow.xml.gz file from one NiFi instance to another and the destination node failed to start. We determined that the origin node had custom NARs loaded containing components that did not exist on the destination node. The error messaging in the nifi-app.log file was not helpful for determining this issue.

I propose improving the error message to be much clearer about what is happening and what specific components are missing, and possibly bringing up the UI with "ghosted" components displayed in the UI (and the flow unavailable to be started until that is resolved).