Fix broken string-formatting syntax
completed by: Jorge Alberto Gómez López
mentors: Walter Bender
(link) There are several places throughout the code that use a comma instead of a % so that the format of error messages and log messages break, e.g.:
if self._activity_cache is not None:
raise ValueError('Activity %s is already tracked',
activity.get_id())
should be:
if self._activity_cache is not None:
raise ValueError('Activity %s is already tracked' %
activity.get_id())