Only release semaphore when acquire succeeds (#128)

Fixes GH-127

internal/worker/worker.go

@@ -90,13 +90,13 @@ func (w *Worker[T]) Do(ctx context.Context, fn WorkFunc[T]) error {
 	}
 
 	if err := w.sem.Acquire(ctx, 1); err != nil {
-		return err
+		return fmt.Errorf("failed to execute job: %w", err)
 	}
 
 	// It's possible the worker was stopped while we were waiting for the
 	// semaphore to acquire, but the worker is actually stopped.
 	if w.isStopped() {
-		w.sem.Release(1)
+		defer w.sem.Release(1)
 		return ErrStopped
 	}
 
@@ -127,8 +127,12 @@ func (w *Worker[T]) Wait(ctx context.Context) error {
 		return ErrStopped
 	}
 
+	if err := w.sem.Acquire(ctx, w.size); err != nil {
+		return fmt.Errorf("failed to wait for all jobs to finish: %w", err)
+	}
 	defer w.sem.Release(w.size)
-	return w.sem.Acquire(ctx, w.size)
+
+	return nil
 }
 
 // Done immediately stops the worker and prevents new work from being enqueued.