More explicit helper messages regarding neurolexID for neuron export

[pgleeson]

As reported by @MRIO, the usage of neurolexID in segment groups is not very explicit. These are used to determine unbranched segments, which get mapped on to NEURON sections.
https://github.com/NeuroML/org.neuroml.export/blob/development/src/main/java/org/neuroml/export/neuron/JSONCellSerializer.java#L301
https://github.com/OpenSourceBrain/SmithEtAl2013-L23DendriticSpikes/blob/e3ce16fdf368a4a39df4b7b0f15a8f1a8b9ca4dc/NeuroML2/L23_NoHotSpot.cell.nml#L12084

This should be better documented, or very specific error messages thrown.

[MRIO]

As background: we commonly had the problem of ion channels not being assigned to segments. When producing executable code this happens completely silently, which endlessly confuses newcomers (and also, some seasoned code writers...)