Improve error messages when PV-relation code is invoked with an empty PV container

There was already an implicit requirement that the creation of PV relations required that >0 PVs had been found. This makes that requirement more explicit in the documentation, and adds an exception in case the requirement is not met.

The underlying cause of the segfault referenced in Moore!439 (merged) was some of this PV-related selection code being executed in events with no PVs. A proper fix will need changes to the Moore configuration; this MR just makes the error messages better when it all goes wrong.

cc: @apearce @ahennequ