middleware: Failure on init causes panic until process restart

[lstoll]

We use a once block to set up the OIDC client in the middleware:

oidc/middleware/middleware.go

Lines 268 to 274 in f864530

	var initErr error
	h.oidcClientInit.Do(func() {
	h.oidcClient, initErr = oidc.DiscoverClient(ctx, h.Issuer, h.ClientID, h.ClientSecret, h.RedirectURL)
	})
	if initErr != nil {
	return nil, initErr
	}

If the discovery fails, this will return an error and never attempt to initialise the client again. We should always try and initialise until we have a functioning client.