def mark_failed(self, clsname, msg):
        '''Marks the tracer as failed with the given exception class name
        :code:`clsname` and message :code:`msg`.

        May only be called in the started state and only if the tracer is not
        already marked as failed. Note that this does not end the tracer! Once a
        tracer is marked as failed, attempts to do it again are forbidden.

        If possible, using the tracer as a context manager (i.e., with a
        :code:`with`-block) or :meth:`.mark_failed_exc` is more convenient than
        this method.

        :param str clsname: Fully qualified name of the exception type that
            caused the failure.
        :param str msg: Exception message that caused the failure.
        '''
        self.nsdk.tracer_error(self.handle, clsname, msg)