Preserve log trace when there is only one error

Noticed this "break of stacktrace" when diagnosing an issue today.

Change-Id: I5e9c200b6cc25c6ef4e25cfd8bc9f746ff6df89d
Reviewed-on: https://skia-review.googlesource.com/c/buildbot/+/757317
Reviewed-by: Eric Boren <borenet@google.com>
Auto-Submit: Kevin Lubick <kjlubick@google.com>
Commit-Queue: Kevin Lubick <kjlubick@google.com>
Reviewed-by: Leandro Lovisolo <lovisolo@google.com>
diff --git a/task_scheduler/go/scheduling/task_scheduler.go b/task_scheduler/go/scheduling/task_scheduler.go
index 3d7b77b..0127b90 100644
--- a/task_scheduler/go/scheduling/task_scheduler.go
+++ b/task_scheduler/go/scheduling/task_scheduler.go
@@ -1437,7 +1437,9 @@
 	s.queue = queue
 	s.lastScheduled = now.Now(ctx)
 
-	if len(errs) > 0 {
+	if len(errs) == 1 {
+		return skerr.Wrapf(errs[0], "Cannot schedule all tasks")
+	} else if len(errs) > 1 {
 		rvErr := "Got failures: "
 		for _, e := range errs {
 			rvErr += fmt.Sprintf("\n%s\n", e)