Add descriptive error message to AudibleTransits.convert_tracks_to_utm

[dbetchkal]

A simple improvement to AudibleTransits.convert_tracks_to_utm() is to handle the AttributeError that occurs if the method is run before AudibleTransits.utm_zone().

This issue considers a marginal use case because most users will use the well-ordered command line interface for the AudibleTransits class. Nevertheless, adding a brief statement such as ".convert_tracks_to_utm() must be run after .utm_zone()" to an except block would be helpful. Alternatively, it may be feasible to combine the two methods.