[tune] Add a time/timesteps since last restore metric (#2819)

* rsm

* always log to avoid changing schema for csv writer

* add iter since restore

* update

* criteria warn

python/ray/tune/trial.py

@@ -199,7 +199,9 @@ class Trial(object):
 
         for criteria, stop_value in self.stopping_criterion.items():
             if criteria not in result:
-                raise TuneError("Stopping Criteria not provided in result.")
+                raise TuneError(
+                    "Stopping criteria {} not provided in result {}.".format(
+                        criteria, result))
             if result[criteria] >= stop_value:
                 return True