Prevent issues when AWS IAM can not be reached

This weekend we had some issues when sts.amazonaws.com could not be reached temporarily. Dinerl corrupted its ETS table `dinerl_data` that holds the session credentials, because it put an error response in this table instead. As `dinerl:update_data/4` expects the ETS contents to be a specific tuple, it also could not recover after the AWS problem went away (it would always hit an exception).

This is my take on addressing the issue. You might wanna incorporate this or similar.

Cheers,
Martin