Dwi BudUt

Results 2 issues of Dwi BudUt

Add log debug when send error message from server to client, the same as was added in `ocppj/client.go` : https://github.com/lorenzodonini/ocpp-go/blob/9b4800337eaee4dddd4996396942fec6191dd8b3/ocppj/client.go#L245

## Proposed changes - Fix type of field `GetCertificateStatusResponse.Status` to `types.Certificate15118EVStatus` instead of `types.GenericStatus` - Add omitempty to `certificateType` field base on doc. spec requirement - Also fix panic nil...