Confirm failed write to Redis are not ignored

[olevski]

A recent incident on limited involved Redis being in a state where each replica of it seemed to be running independently. This lead to the ui-server and the gateway not being able to save their sessions in Redis.

But during this time we could only see failed login attempts and no keys in Redis. But there were no specific errors that indicated in the gateway logs that:

1. The connection to redis was bad
2. The writes to redis were bad

Therefore we should confirm that the gateway properly handles:

• when redis is running correctly and then dies
• when redis is not available as Renku and similar components are starting up
• when writes to redis fail