Add remediation result status and timestamp to the rule_evaluation_status table

[jhrozek]

In order to let the user know if/why a remediation had failed, we need to extend the rule_evaluation_status table with a new column. This could be a simple text column where we would add a textual representation of the error.

We'll also have to extend the way the policy engine returns errors, currently just the evaluation result is returned as a single instance of error, we probably need both evaluation and remediation results.