Wrong input should result into an exception

The code in spdx.py function `is_compound_expression` may run into an exception for several cases, and this results into a `return False`.

This is semantically overloaded.
In case of `False` we cannot distinguish between
- the value cannot be interpreted at all (invalid characters)
- it is a compound expression, but consists of partially unknown, but maybe valid simple (e.g. LicenseRef-*) expression(s)

So it may not be clear what is feasible of how to proceed in case of False.

In case of invalid characters the exception should not be caught, but (re)raised so that an appropriate exception handling can be applied.

The `validate` parameter should be made available in the `is_compound_expression` function to allow either both license item validation along with the compound check or compound check only.

spdx.py の is_compound_expression から始め、無効な文字と一部が不明なライセンス式に対する例外処理を追跡します。validate パラメーターが他の場所でどのように扱われているかを確認し、無効な入力では適切な例外が伝播するようにしつつ、ライセンス項目の検証あり・なしのどちらでも複合式をチェックできるようにします。呼び出し側が、無効な入力と、不明な項目を含む有効な複合式を区別できれば完了です。

索引モデルが issue の本文から書いたものです。