Reconsider exception on FbAttachment.close() if currently not open

[mrotteveel]

Currently, FbAttachment.close() requires an exception to be thrown if not currently connected. We should reconsider this design as it makes it hard to use it in try-with-resources, in cases where the attachment might be closed by other means (e.g. in FBManager when dropping a database), or the attachment failed to connect.