Most error handling code in mongocxx throws an exception derived from mongocxx::exception, such as mongocxx::logic_error, mongocxx::operation_exception, etc. We should audit the codebase to find the remaining functions that throw mongocxx::exception directly, and replace the exceptions in those throw statements with the appropriate specific exception.

Also, consider whether gridfs_exception deserves to derive from exception, as it does now, or if it should be a subclass of operation_exception, or just deleted in favor of the existing exceptions.