suggestion: Feels a little odd to raise here on a `join` function.

[huangh]

          suggestion: Feels a little odd to raise here on a `join` function. 

Could be slightly more concise to raise in events_gtfs_rt.generate_gtfs_rt_events() and claify in the doc string that this function should always return results.

Originally posted by @rymarczy in #519 (comment)