dispatcher: correctly wrap errors
dispatcher: correctly wrap errors

Use `serrors.WrapStr` instead of `serrors.New` in the dispatcher code when
wrapping errors. Otherwise the errors are logged as `"err": {}`, because of JSON
marshalling,

GitOrigin-RevId: e9e897a7307bb865810173a26a5ff1e26ff47b95

netsec-ethz/scion,netsec-ethz/scion,netsec-ethz/scion,netsec-ethz/scion