Improve Cancellation Semantics

[kraftp]

Allow cancelling children (dbos-inc/dbos-transact-py#701) and make cancellation a terminal state (dbos-inc/dbos-transact-py#703)

[devhawk]

Also as part of this work, make sure we are throwing the correct exception (DBOSAwaitedWorkflowCancelledException vs DBOSWorkflowCancelledException)

Maybe also rename one of these to make them easier to disambiguate?

[devhawk]

In python, DBOSWorkflowCancelledError derives from BaseException and DBOSAwaitedWorkflowCancelledError derives from Exception.

DBOSWorkflowCancelledError is raised:

• SysDB _check_operation_execution_txn
• _run_preemptible_step

DBOSAwaitedWorkflowCancelledError is raised

• sysdb check_workflow_result and update_workflow_outcome
• _get_wf_invoke_func (transforms DBOSWorkflowCancelledError -> DBOSAwaitedWorkflowCancelledError

from https://docs.python.org/3/tutorial/errors.html:

BaseException is the common base class of all exceptions. One of its subclasses, Exception, is the base class of all the non-fatal exceptions. Exceptions which are not subclasses of Exception are not typically handled, because they are used to indicate that the program should terminate. They include SystemExit which is raised by sys.exit() and KeyboardInterrupt which is raised when a user wishes to interrupt the program.

[devhawk]

Note children cancellation was tracked by #408 and implemented by #423. Keeping issue open to track making cancellation a terminal state and ensuring consistent cancellation exception semantics