`DELETE` events are dropped silently when error happens in `CREATE` or `UPDATE` events in `sync.Solve`

[randmonkey]

From investigation of FTI-6816, I found that when CREATE or UPDATE events meets errors, DELETE events are not handled without any output or errors mentioning it. This silent failure caused KIC failed to delete routes for deleted HTTPRoute in Konnect when it meets error in creating targets and lack of error logs caused confusion on investigation of the issue.

[randmonkey]

After discussion, we decide not to change the behavior of the runners since continue the operation on errors may cause inconsistencies or even downtimes. But we should return the events dropped due to errors to the callers to provide necessary logs.