log#warn and include contextual info

Author: yasunariw

lib/action.ml

@@ -247,12 +247,12 @@ module Action (Github_api : Api.Github) (Slack_api : Api.Slack) = struct
             match%lwt State.save ctx.state path with
             | Ok () -> Lwt.return_unit
             | Error msg ->
-              log#error "%s" msg;
+              log#warn "failed to save state file %s : %s" path msg;
               Lwt.return_unit)
         in
         Lwt.return_some user_id
       | Error msg ->
-        log#error "%s" msg;
+        log#warn "failed to query slack auth.test : %s" msg;
         Lwt.return_none
     in
     let process link =