cloud/storage/dropbox: fix build

Change-Id: I46deb0a9b71fa5bb770eed8fd79399f073041529
Reviewed-on: https://upspin-review.googlesource.com/14700
Reviewed-by: Rob Pike <r@golang.org>
diff --git a/cloud/storage/dropbox/dropbox_test.go b/cloud/storage/dropbox/dropbox_test.go
index 7469369..495a424 100644
--- a/cloud/storage/dropbox/dropbox_test.go
+++ b/cloud/storage/dropbox/dropbox_test.go
@@ -106,7 +106,7 @@
 func token() (string, error) {
 	tokenFile := path.Join(os.TempDir(), "upspin-test-token")
 
-	token, _ := ioutil.ReadFile(tokenFile)
+	token, err := ioutil.ReadFile(tokenFile)
 	if err == nil {
 		return string(token), nil
 	}