tasks: add pending field

this indicates if a task is scheduled. previously, we relied
on task.progress being 1

src/taskworker.js

@@ -33,8 +33,8 @@ const TASKS = { // indexed by task type
     syncDyndns: dyndns.sync,
     updateDiskUsage: system.updateDiskUsage,
 
-    _identity: async (arg, progressCallback) => { progressCallback({}); return arg; },
-    _error: async (arg, progressCallback) => { progressCallback({}); throw new Error(`Failed for arg: ${arg}`); },
+    _identity: async (arg, progressCallback) => { progressCallback({ percent: 20 }); return arg; },
+    _error: async (arg, progressCallback) => { progressCallback({ percent: 20 }); throw new Error(`Failed for arg: ${arg}`); },
     _crash: (arg) => { throw new Error(`Crashing for arg: ${arg}`); }, // the test looks for this debug string in the log file
     _sleep: async (arg) => setTimeout(process.exit, arg)
 };