Raise actually informative error if conditions passed to RowSelector result in non-array

[kspurgin]

Without this, we get an uninformative mess like:

/Users/kristina/code/mig/kiba-extend/lib/kiba/extend/utils/fieldset.rb:51:in `fetch': no implicit conversion of Symbol into Integer (TypeError)
	from /Users/kristina/code/mig/kiba-extend/lib/kiba/extend/utils/fieldset.rb:51:in `block in get_field_values'
	from /Users/kristina/code/mig/kiba-extend/lib/kiba/extend/utils/fieldset.rb:49:in `each'
	from /Users/kristina/code/mig/kiba-extend/lib/kiba/extend/utils/fieldset.rb:49:in `get_field_values'
	from /Users/kristina/code/mig/kiba-extend/lib/kiba/extend/utils/fieldset.rb:35:in `block in populate'
	from /Users/kristina/code/mig/kiba-extend/lib/kiba/extend/utils/fieldset.rb:35:in `each'
	from /Users/kristina/code/mig/kiba-extend/lib/kiba/extend/utils/fieldset.rb:35:in `populate'
	from /Users/kristina/code/mig/kiba-extend/lib/kiba/extend/transforms/merge/multi_row_lookup.rb:88:in `block in process'

Example of bad conditions lambda:

conditions: ->(_r, rows) do
  res = rows.select{ |row| row[:contype] == 'Organization' }
  res.empty? ? res :  res.first
end