No thorough error handling on appending to the event log

[louiseschmidtgen]

We are not properly handling this error:

rev, err = l.log.Append(ctx, deleteEvent)
if err != nil {
// If error on Append we assume it's a UNIQUE constraint error, so we fetch the latest (if we can)
// and return that the delete failed

From: https://github.com/canonical/k8s-dqlite/blob/master/pkg/kine/logstructured/logstructured.go#L221

This can lead to issues as captured in: #135