Fix DataCloneError when remote extension throws error in Firefox

src/dispatch/shared-dispatch.js

@@ -204,7 +204,7 @@ class SharedDispatch {
             } else {
                 promise.then(
                     result => worker.postMessage({responseId: message.responseId, result}),
-                    error => worker.postMessage({responseId: message.responseId, error})
+                    error => worker.postMessage({responseId: message.responseId, error: `${error}`})
                 );
             }
         }