Add isClosed and exception handling for closed state to oauth2.Client

[illaz]

Until now there is no way to detect whether an oauth2.Client is closed or not. This PR changes that.

There is now an isClosed field on the oauth2.Client.

A request on a closed client no longer leads to a 'Null check operator used on a null value' error, but instead an http.ClientException is thrown with a meaningful error message.

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