Stop ignoring semaphore errors in tower-batch

[teor2345]

Is your feature request related to a problem? Please describe.

tower-batch ignores semaphore errors, but tower::Buffer handles them:
https://github.com/ZcashFoundation/zebra/blob/main/tower-batch/src/service.rs#L97
https://docs.rs/tower/0.4.4/src/tower/buffer/service.rs.html#124

Describe the solution you'd like

We should stop returning Poll::Ready(Ok(())) when the semaphore errors.

Instead, we should return the semaphore errors, mapping them as needed:

        self.semaphore
            .poll_acquire(cx)
            .map_err(|_| self.get_worker_error())

Describe alternatives you've considered

Do nothing: the code hides any semaphore errors, but continues to fail internally.

Additional context

These hidden failures could be causing hangs, so this issue is a high priority.

This issue was discovered during the review in #1593. It could be a cause of the hangs or CPU usage in #1435 or #1634, so it's a high priority.