[bugfix] Return useful err on server start failure

internal/scheduler/scheduler.go

@@ -44,6 +44,11 @@ func (sch *Scheduler) Start() bool {
 	return false
 }
 
+// Started returns true if the scheduler has been started.
+func (sch *Scheduler) Started() bool {
+	return sch.ts != nil
+}
+
 // Stop attempts to stop scheduler, cancelling
 // all running tasks. Returns false if not running.
 func (sch *Scheduler) Stop() bool {