Morecast: Failing to calculate indices for one station fails for all stations

**Actual Behaviour**

When a station in a station group fails to calculate, all other stations fail as well, and the frontend displays no rows

**Steps To Reproduce**

1. Remove weather params for a date for a station in a station group
2. See failure in API
3. See no rows rendered on frontend

**Screenshots**

If applicable, add screenshots to help explain your problem.

**Expected Behaviour**

A failure for a station shouldn't cause other stations to not render

Start with the API request from the reproduction steps after removing weather parameters for one station, and trace the station-group index calculation failure. Then inspect the frontend path that renders the station rows. Done means one station's failure is handled without preventing other stations from returning or rendering.

Written by the indexing model from the issue text.