UX enhancement when Vault is unavailable

Hello,

I would like to suggest a basic UX enhancement related to the way Onyxia deals with errors coming from Vault (possibly S3 as well, though that is untested).

When the Vault configured for Onyxia is down, the UI just hangs on the spinner, without any feedback for the user. Since Vault is heavily relied upon (especially for storing many bits of Onyxia related config), it's probably unrealistic to design a fail-safe error handling. But it could be beneficial to at least acknowledge the error and warn the user, even with a basic alert (e.g. : *A system Onyxia depends on is currently unavailable or failing. As such, Onyxia cannot be used at the moment.*).

I'm not sure of the behavior when the S3 storage is unavailable, but maybe this suggestion could also be applied to a failure on that end.

Thanks !