Mistral Error Handling

We need a mechanism in Mistral that would allow to define routines for error handling. For example, if we started a workflow and some task of a workflow failed we need to be able to:
- Define a number of retries for the task and delays between them (i.e. fixed delays, exponential etc.)
- Define a revert action for the task in case all retries have been exhausted
- Error propagation policy for the task (whether it should be propagated to tasks that depend on it)