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

assigned to @mstahl

added Filters label

added 1 deleted label

• [2020-03-24 00:07] Validation started with lhcb-test-throughput#483
• [2020-03-26 00:12] Validation started with lhcb-test-throughput#484
• [2020-03-31 00:06] Validation started with lhcb-test-throughput#485
• [2020-04-01 00:09] Validation started with lhcb-test-throughput#486
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• [2020-04-07 00:08] Validation started with lhcb-test-throughput#489
• [2020-04-08 01:16] Validation started with lhcb-test-throughput#490
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• [2020-04-11 00:10] Validation started with lhcb-test-throughput#493
• [2020-04-13 00:08] Validation started with lhcb-test-throughput#494
• [2020-04-14 00:11] Validation started with lhcb-test-throughput#495
• [2020-04-15 00:17] Validation started with lhcb-test-throughput#496
• [2020-04-17 00:07] Validation started with lhcb-test-throughput#497
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• [2020-04-21 00:07] Validation started with lhcb-test-throughput#499
• [2020-04-22 00:07] Validation started with lhcb-test-throughput#500
• [2020-04-22 12:12] Validation started with lhcb-master-mr#702
• [2020-04-23 00:06] Validation started with lhcb-test-throughput#501

mentioned in issue Moore#148 (closed)

assigned to @mzdybal and unassigned @mstahl

assigned to @dovombru

unassigned @mzdybal

mentioned in issue Moore#158 (closed)

/ci-test

assigned to @olupton and unassigned @dovombru

resolved all threads

assigned to @dovombru and unassigned @olupton

assigned to @rmatev

unassigned @dovombru

• [2020-04-24 00:10] Validation started with lhcb-test-throughput#502
• [2020-04-25 00:08] Validation started with lhcb-test-throughput#503

added Functors enhancement labels and removed Filters + 1 deleted label

approved this merge request

merged

unassigned @rmatev

mentioned in commit fb7183b9

mentioned in issue Moore#163 (closed)