Improve error handling and helpers for failed tasks

[ybastide]

Context

We noticed it would be helpful, when a workflow fails, to have more information on the underlying activity and error details to provide helpful information to help diagnosing why the workflow failed.

Tech details

Current approach

When handling a failure in a workflow, we have access to the failure context via simpleflow.task.TaskFailureContext, which exposes:

• the underlying exception: failure_context.exception
• helper properties to cache and access selected exception details: failure_context.{task_name, task_error, task_error_type}

Possible improvements

It would be helpful to have access to additional information regarding the underlying exception in a straightforward way, with all fields at the same level, e.g.:

• directly access the exception attribute (without extra caching) and its own attributes
• provide the exception context and details:
  • failed task name
  • exception type
  • exception message (reason)
  • extra details specific to TaskFailed exceptions:
    • error
    • error type

The resulting improvements should be documented under the Features / Error Handling section.