Better handling of invalid GTFS

[antoine-de]

For the moment invalid gtfs remove the dataset from the API at startup.
If the GTFS becomes invalid while reloading, the old one is kept (and a sentry error is issued).

There are several flows in this approach:

startup

We want at least to keep the invalid dataset and mark it as invalid in the / api.
An even nicer approach would be to mark it as invalid at try again to load it later (once a day ?). This would imply a bigger refactoring of the code since the DatasetActor would return an option (or result), but I think it would be not that bad.

We should also check if the sentry message marking an invalid dataset is send, I don't see them on sentry.

base schedule reloading

Retrying every 5mn seems a bit overkill 🤔 Maybe we should have an exponential backof, and a limit afterward ?
It seems way less important than the previous point though.