do not update IAM session token with error responses

When the connect to sts.amazonaws.com fails, the session token in the
dinerl_data ETS table would be overwritten with the AWS error response.
This fails all subsequest requests and, worse, also prevents
dinerl:update_data/4 to ever succeed again (badmatch on line 253).
reschedule dinerl:update_data/4 after it has completed

Prevent piling up update_data calls with timer:apply_interval when
the iam:get_session_token/2 call takes long or fails altogether.