def raise_on_nil_instance_vars(object, vars)
    nil_vars = nil_instance_vars(object, vars)
    unless nil_vars.empty?
      raise ObjectValidationError.new("The following instance variables were nil: #{nil_vars.join(', ')}.")
    end
  end
def raise_on_nil_instance_vars(object, vars)
    nil_vars = nil_instance_vars(object, vars)
    unless nil_vars.empty?
      raise ObjectValidationError.new("The following instance variables were nil: #{nil_vars.join(', ')}.")
    end
  end
For each symbol in vars, checks to see that the corresponding instance
 variable in the specified object is not nil.
 If any are nil, raises an error listing the nils.