Nservicebus: InsertAfterIfExists and InsertBeforeIfExists do not throw an exception if the specified anchoring step is in a different stage of the pipeline

[SzymonPobiega]

[timbussmann]

Checked against current v6 and it seems to throw the following exception when trying to insertbefore/after a behavior in a different stage:

Registration '' specified in the insertafter of the '' step does not exist. Current StepIds: ''

The exception message could be a bit more helpful, but it should stop the endpoint from starting with invalid behavior dependencies.

I think we can close this? @Particular/nservicebus-maintainers @SzymonPobiega