The usual convention to create new object is to prefix `New*` so
this commit renames the `WorkerClientError`. Initially I thought
it would be `NewWorkerClientError()` but looking at the package
prefix it seems unneeded, i.e. `clienterrors.New()` already
provides enough context it seems and it's the only error we
construct.
We could consider renaming it to `clienterror` (singular) too
but that could be a followup.
I would also like to make `clienterror.Error` implement the
`error` interface but that should be a followup to make this
(mechanical) rename trivial to review.
