Dataset detail page `/{id}` currently returns 502 if static data is unavailable

[thbar]

(Following work on #109/#107).

If one queries /{id} when the base schedule (GTFS) is not available, they will get a 502 with an error.

For better ergonomics, we could instead generate a degraded version of the detail page (e.g. https://tr.transport.data.gouv.fr/reseau-de-transports-en-commun-de-la-communaute-dagglomeration-de-lauxerrois/) filling what we can.

I believe most of what is generated could be still generated, actually.

To be discussed!