fix: add handling for retrieve estimates on new shuttle creation

[meagharty]

Summary of changes

Asana Ticket: Follow up on "🏹 Implement "Create/Edit Shuttle Route Definition" - Time to Next Stop"

• Add fetching of stop data (lat/long) when the initial shuttle hasn't been created yet
• Add explicit error handling for when there is only one stop defined for a direction

Part of this is similar to what the Map component does (Ecto.assoc_loaded? & calling preload) but if those values aren't set yet, then this uses a virtual field display_stop mirroring display_stop_id. This isn't the most elegant solution, opening this as a draft for now.

Reviewer Checklist

• Meets ticket's acceptance criteria
• Any new or changed functions have typespecs
• Tests were added for any new functionality (don't just rely on Codecov)
• This branch was deployed to the staging environment and is currently running with no unexpected increase in warnings, and no errors or crashes.

[meagharty]

I'm not sure we really need both display_stop and display_stop_id but it reduces the changes required across other pieces of work.

[lemald]

I think it's useful to have both because, in the case of an Arrow stop, the display_stop_id is the value from the stop_id column, not the actual database primary key. We could definitely work around that with some helper function to get the display ID for a given display_stop, but I'm not sure how that would play along with form rendering. Anyhow, long story short I think it's fine to keep it this way.

[lemald]

There's the minor thing about the type on display_stop, but otherwise looks good to me! 👍

[lemald]

👍