[FEATURE] Desugarer should not throw IllegalArgumentException

[konnov]

Surfaced in #469. Desugarer should throw an error that is processed via the normal error processing.

 private def flattenTuples(ex: TlaEx): Seq[TlaEx] = ex match {
    case OperEx(TlaFunOper.tuple, args @ _*) =>
      args.map(flattenTuples).reduce(_ ++ _)

    case NameEx(_) =>
      Seq(ex)

    case _ =>
      throw new IllegalArgumentException("Expected a variable or a tuple of variables, found: " + ex)
  }

[konnov]

We should process the default case as e' = e.