Fix nested retry with _Serial flows
 - Major cleanup of _Serial flow _check()
   - simplified, now more closely mimicks _Parallel flow _check()
Improved reporting on unchecked jobs
Cleanup -
  - Let _prepare_to_invoke() handle tried_times and total_tried_times updating
Tests -
 - Test that serial flow jobs are invoked after previous job finishes
 - Fixed mock api status message to match real api status
 - Fixed reporting test to match
 - Assert lines in text in-order
 - More tests