repository: Remove RunWorkers, report ctx.Err()

This removes RunWorkers, which had become mere overhead by successive
refactors. It also ensures that each former user of that function
returns any context error that occurs, so failure to complete an
operation is always reported as an error.

internal/repository/master_index.go

@@ -365,7 +365,7 @@ func (mi *MasterIndex) Save(ctx context.Context, repo restic.Repository, packBla
 					select {
 					case ch <- newIndex:
 					case <-ctx.Done():
-						return nil
+						return ctx.Err()
 					}
 					newIndex = NewIndex()
 				}
@@ -397,10 +397,9 @@ func (mi *MasterIndex) Save(ctx context.Context, repo restic.Repository, packBla
 	}
 
 	// run workers on ch
-	wg.Go(func() error {
-		return RunWorkers(saveIndexParallelism, worker)
-	})
-
+	for i := 0; i < saveIndexParallelism; i++ {
+		wg.Go(worker)
+	}
 	err = wg.Wait()
 
 	return obsolete, err