fix: error info formatting

closes #485

lib/cache-backend-fs.js

@@ -196,7 +196,7 @@ FSBackend.prototype.setItem = function(queueObject, data, callback) {
                     // Just overwrite the file...
                     writeFileData(currentPath, data);
                 } else {
-                    throw new Error("Cache storage of resource (%s) blocked by file: %s", queueObject.url, currentPath);
+                    throw new Error(`Cache storage of resource (${queueObject.url}) blocked by file: ${currentPath}`);
                 }
             }
         } else if (count === pathStack.length - 1) {