Fix lint (#357)

Fix lint left in
https://github.com/bazelbuild/remote-apis-sdks/pull/340
diff --git a/go/pkg/cache/singleflightcache.go b/go/pkg/cache/singleflightcache.go
index b4fe8b9..1a1db5c 100644
--- a/go/pkg/cache/singleflightcache.go
+++ b/go/pkg/cache/singleflightcache.go
@@ -35,6 +35,11 @@
 }
 
 // Load is similar to a sync.Map.Load.
+//
+// Callers must check loaded before val and err. Their value is meaninful only
+// if loaded is true. The err is not the last returned value because it would
+// likely lead the reader to think that err must be checked before loaded.
+//lint:ignore ST1008 loaded must be last, see above.
 func (s *SingleFlight) Load(key interface{}) (val interface{}, err error, loaded bool) {
 	s.mu.RLock()
 	defer s.mu.RUnlock()
diff --git a/go/pkg/cas/upload.go b/go/pkg/cas/upload.go
index ae79d81..b09bfe6 100644
--- a/go/pkg/cas/upload.go
+++ b/go/pkg/cas/upload.go
@@ -147,7 +147,7 @@
 	// TODO(nodir): cache this syscall too.
 	info, err := os.Lstat(ps.Path)
 	if err != nil {
-		return digest.Digest{}, err
+		return digest.Digest{}, errors.WithStack(err)
 	}
 
 	key := makeFSCacheKey(ps.Path, info.Mode().IsRegular(), ps.Exclude)
@@ -155,7 +155,7 @@
 	case !loaded:
 		return digest.Digest{}, errors.Wrapf(ErrNoDigest, "digest not found for %#v", ps)
 	case err != nil:
-		return digest.Digest{}, err
+		return digest.Digest{}, errors.WithStack(err)
 	default:
 		return digest.NewFromProtoUnvalidated(val.(*digested).digest), nil
 	}
@@ -232,7 +232,8 @@
 			}
 		}
 	})
-	return &UploadResult{Stats: u.stats, u: u}, eg.Wait()
+
+	return &UploadResult{Stats: u.stats, u: u}, errors.WithStack(eg.Wait())
 }
 
 // uploader implements a concurrent multi-stage pipeline to read blobs from the