Throw when lease not acquired. This can happen then the rate limiter …

…doesn't allow queuing or is generally wrongly configured

projects/RabbitMQ.Client/Impl/Channel.PublisherConfirms.cs

@@ -283,6 +283,11 @@ await _confirmSemaphore.WaitAsync(reason.CancellationToken)
                         lease = await _outstandingPublisherConfirmationsRateLimiter.AcquireAsync(
                             cancellationToken: cancellationToken)
                             .ConfigureAwait(false);
+
+                        if (!lease.IsAcquired)
+                        {
+                            throw new InvalidOperationException("Could not acquire a lease from the rate limiter.");
+                        }
                     }
                 }