TMSS-483: processed review comments about state transitions and retrying...

...MSS/backend/services/scheduling/lib/subtask_scheduling.py

SAS/TMSS/backend/src/tmss/exceptions.py

SAS/TMSS/backend/src/tmss/tmssapp/subtasks.py

SAS/TMSS/backend/test/t_scheduling.py

SAS/TMSS/backend/test/tmss_test_data_rest.py

SAS/TMSS/client/lib/tmss_http_rest_client.py