GCS: report error when we fail to get access token

Author: walac

src/cache/gcs.rs

@@ -62,7 +62,15 @@ impl Bucket {
         let client = self.client.clone();
 
         let creds_opt_future = if let &Some(ref cred_provider) = cred_provider {
-            future::Either::A(cred_provider.credentials(&self.client).map(Some))
+            future::Either::A(
+                cred_provider
+                    .credentials(&self.client)
+                    .map_err(|err| {
+                        warn!("Error getting credentials: {:?}", err);
+                        err
+                    })
+                    .map(Some),
+            )
         } else {
             future::Either::B(future::ok(None))
         };