def error(wi_or_fei)

      fei = Ruote.extract_fei(wi_or_fei)
      err = @context.storage.get('errors', "err_#{fei.sid}")

      err ? ProcessError.new(err) : nil
    end
def error(wi_or_fei)

      fei = Ruote.extract_fei(wi_or_fei)
      err = @context.storage.get('errors', "err_#{fei.sid}")

      err ? ProcessError.new(err) : nil
    end
Given a workitem or a fei (or a String version of a fei), returns
 the corresponding error (or nil if there is no other).