fix(kuma-cp): unnecessary `failed to write error to closed channel` errors

[lobkovilya]

Motivation

Error channel had capacity 0, that's why every time when we write to it like:

select {
case errChan <- err:
default:
    log.Error(err, "failed to write error to closed channel")
}

we always go to the default section.

Implementation information

Update channel capacity to be 1.

Supporting documentation

None

[lobkovilya]

I checked more carefully, we don't need errCh in callbacks at all, should be simpler now. It should be clear now that capacity 1 is enough