feat(ScheduleController): smaller schedules_for_stop response + logging

[thecristen]

Summary of changes

Asana Ticket: Stop pages intermittently missing Commuter Rail schedules

I happened to be able to consistently reproduce the problem for a particular bus route on a particular stop on prod... but ultimately I never figured out a definitive cause for this. Splunk didn't reveal any errors. No network problems or console errors. I suspect that caching might exacerbate the problem and cause the backend to return [] instead of re-trying within the cache max_age window. But still, ideally it wouldn't erroneously return [] in the first place.

This PR does 3 things:

1. Returns an error response if the Schedules.Repo.schedules_for_stop/2 call returns {:error, error} value. An error will be logged in Splunk and the frontend would handle it by showing the error view with the bus gif.
2. Logs a message if the list of schedules returned happens to be empty. Just for informational/future debugging purposes should be issue persist and not be directly related to a Schedules.Repo error.
3. Dramatically reduces the size of the JSON response for SiteWeb.ScheduleController.schedules_for_stop/2 in hopes of improving performance. It removes data that's not used on the frontend.

Comparison for Alewife Station (/api/stops/place-alfcl/schedules?last_stop_departures=false&future_departures=true), using Chrome DevTools with no caching and "Fast 3G" network throttling setting:

	Before	After	Difference
Response size	272 kB	53.4 kB	-80%
Time	2.49s	885ms	-64%

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