Fixes #7406 UnicodeDecodeError in on_tracker_error_alert

[kozlovsky]

This PR fixes #7406 by moving the self._logger.error call into the try/except block. It also replaces self._logger.exception with self._logger.warning in case of an error, as the error's origin is known and handled adequately by the workaround.

Initially, in the discussion of #7410, I suggested adding a safe_repr function to wrap alert with it, but according to arvidn/libtorrent#143, on_tracker_error_alert is the only place where the error can happen, so it is not necessary to implement a reusable function for safe_alert, as other on_xyz_alert handlers do not require a fix.