Improve error when nonhierarchical relation row is missing an ID

[kspurgin]

Currently it is this, which does show the error, but it's buried in undefined method message:

ERROR in CollectionspaceMigrationTools::Csv::BatchProcessor.process
MESSAGE:
undefined method `each' for #<CollectionSpace::Mapper::Response:0x0000000113602818 @handler=<#CollectionSpace::Mapper::HandlerFullRecord:12310 nonhierarchicalrelationship https://westerville.staging.collectionspace.org/cspace-services>, @orig_data={"item2_id"=>"O1049021", "item1_type"=>"movements", "item2_type"=>"collectionobjects"}, @errors=["required field missing: item1_id must be present"], @warnings=[], @merged_data={"item2_id"=>"O1049021", "item1_type"=>"movements", "item2_type"=>"collectionobjects", "relationshiptype"=>"affects"}, @states=[:defaults_merged, :merged_keys_downcased, :validated], @xpaths=nil, @split_data={}, @transformed_data={}, @combined_data={}, @doc=nil, @terms=[], @identifier=""> IN /Users/kristina/code/cs/migration_tools/lib/collectionspace_migration_tools/csv/row_processor.rb:45:in `handle_failure'