Don't fail if task dirs don't exist on creation

Task dir metadata is created in AllocRunner.Run which may not run before
an alloc is sync'd and Nomad exits. There's no reason not to just create
task dir metadata on restore if it doesn't exist.

client/alloc_runner.go

@@ -284,10 +284,11 @@ func (r *AllocRunner) RestoreState() error {
 
 		td, ok := r.allocDir.TaskDirs[name]
 		if !ok {
-			err := fmt.Errorf("failed to find task dir metadata for alloc %q task %q",
-				r.alloc.ID, name)
-			r.logger.Printf("[ERR] client: %v", err)
-			return err
+			// Create the task dir metadata if it doesn't exist.
+			// Since task dirs are created during r.Run() the
+			// client may save state and exit before all task dirs
+			// are created
+			td = r.allocDir.NewTaskDir(name)
 		}
 
 		tr := NewTaskRunner(r.logger, r.config, r.stateDB, r.setTaskState, td, r.Alloc(), task, r.vaultClient, r.consulClient)