SSL_shutdown error codes are un-discoverable

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The _raise_current_error in Connection.shutdown is incorrect because SSL_shutdown can result in SSL_ERR_WANT_WRITE or SSL_ERR_WANT_READ. The way you discover this, according to the official documentation, is:

If the underlying BIO is non-blocking, SSL_shutdown() will also return when the underlying BIO could not satisfy the needs of SSL_shutdown() to continue the handshake. In this case a call to SSL_get_error() with the return value of SSL_shutdown() will yield SSL_ERROR_WANT_READ or SSL_ERROR_WANT_WRITE.

The current behavior is that the error code is masked because the return code is lost.

[exarkun]

(stupid comment deleted)