Return string when storage.sync.get() promise fails

Related issue:
- https://github.com/uBlockOrigin/uBlock-issues/issues/2660

platform/common/vapi-background.js

@@ -1627,7 +1627,7 @@ vAPI.cloud = (( ) => {
         try {
             bin = await webext.storage.sync.get(keys);
         } catch (reason) {
-            return reason;
+            return String(reason);
         }
         let chunkCount = 0;
         for ( let i = 0; i < maxChunkCountPerItem; i += 16 ) {