error for list when list of lists is provided should be more explicit

[lzehl]

We need to better inform about the error for lists when the user accidentally provides a list of lists.

[apdavison]

This is the message currently provided by validate():

import openminds.v4 as om

mouse = om.controlled_terms.Species.mus_musculus
# accidentally provide list of lists
dsv = om.core.DatasetVersion(study_targets=[[mouse]])
dsv.validate(ignore=["required"])

{'type': ['study_targets: Expected AuditoryStimulusType, BiologicalOrder, BiologicalSex, BreedingType, CellCultureType, CellType, Disease, DiseaseModel, ElectricalStimulusType, GeneticStrainType, GustatoryStimulusType, Handedness, MolecularEntity, OlfactoryStimulusType, OpticalStimulusType, Organ, OrganismSubstance, OrganismSystem, Species, SubcellularEntity, TactileStimulusType, TermSuggestion, TissueSampleType, UBERONParcellation, VisualStimulusType, CustomAnatomicalEntity, ParcellationEntity, ParcellationEntityVersion, value contains list']}

Is this ok, or should it give more information?