ENH: Preserve greedy execution with provisional tasks #985

Is your feature request related to a problem?

When provisional task creation fails, a new DAG cannot be created and therefore execution cannot continue. The pytask session will stop.

Relevant paths include:

• provisional.py: failures while collecting generated tasks, running pytask_collect_modify_tasks, or validating task identities.
• provisional_utils.py: failures in recreate_dag, including dependency resolution, cycle detection, duplicate products, or scheduler rebuilding.
• recreate_dag also assigns session.dag before rebuilding the scheduler. If scheduler rebuilding fails, the session may contain a new DAG paired with an old scheduler.

Describe the solution you'd like

A better behavior would be to handle the whole process like a transaction and when any of the steps of registering these new provisional tasks fails, they are collected as an execution error and execution continues with the old DAG.