GBFS: Correctly handle errors during provider updates

[pablohoch]

Errors that occur when updating a provider (connection errors or invalid responses) are now correctly handled, reported and don't cause other providers to disappear. The existing provider data is kept and will continue to be used for routing when an error occurs during the update.

Previously, errors were only handled correctly during the initial processing of a provider.

Note: If a provider is unreachable or doesn't return a valid discovery file during MOTIS startup, it won't be available even if it becomes reachable later on (until MOTIS is restarted).

This should hopefully fix public-transport/transitous#988