better error code

Signed-off-by: Jess Frazelle <acidburn@jessfraz.com>
diff --git a/filter.go b/filter.go
index 49ba783..aaa700b 100644
--- a/filter.go
+++ b/filter.go
@@ -128,7 +128,7 @@
 			"criteria": fmt.Sprintf("%#v", fltr.Criteria),
 		}).Debug("adding Gmail filter")
 		if _, err := api.Users.Settings.Filters.Create(gmailUser, &fltr).Do(); err != nil {
-			return fmt.Errorf("creating filter failed: %v", err)
+			return fmt.Errorf("creating filter [%#v] failed: %v", fltr, err)
 		}
 	}
 
diff --git a/go.mod b/go.mod
index 783df49..e7b1ae6 100644
--- a/go.mod
+++ b/go.mod
@@ -12,3 +12,5 @@
 	google.golang.org/api v0.0.0-20181229000844-f26a60c56f14
 	google.golang.org/appengine v1.4.0 // indirect
 )
+
+go 1.13