Add error return to PKCS11Session.Close (breaking change)

[dmjones]

During the work for #24, it became clear that PKCS11Session.Close() ought to return an error but doesn't.

This would be a breaking change, so the interim fix was to add an alternative PKCS11Session.CloseSession() method that does return the error. The PKCS11Session.Close() was deprecated.

During the next breaking change release, we should either remove the Close() method or change its signature.