[sk] Add reminder for authenticating locally

Change-Id: Ib55094917b64ca0b5ccd9934278d98cb97a2a033
Reviewed-on: https://skia-review.googlesource.com/c/buildbot/+/561176
Reviewed-by: Ravi Mistry <rmistry@google.com>
Auto-Submit: Kevin Lubick <kjlubick@google.com>
diff --git a/sk/go/asset/asset.go b/sk/go/asset/asset.go
index 85c70ad..5fb904f 100644
--- a/sk/go/asset/asset.go
+++ b/sk/go/asset/asset.go
@@ -299,7 +299,7 @@
 		ts, err = luciauth.NewLUCIContextTokenSource(auth.ScopeUserinfoEmail)
 	}
 	if err != nil {
-		return nil, skerr.Wrap(err)
+		return nil, skerr.Wrapf(err, "Could not get token source. \nTry running 'gcloud auth application-default login'\n")
 	}
 	httpClient := httputils.DefaultClientConfig().WithTokenSource(ts).Client()
 	cipdClient, err := cipd.NewClient(httpClient, rootDir, cipd.DefaultServiceURL)