Fix possibility of non atomic promise+task creation for CreatePromiseAndTask

[avillega]

Problem:

In some instances it is possible to end up with the creation of and claim of a task where the promise was not created. This happens because on conflict for promise creation we do nothing and continue as if the promise was created.

Describe the solution you'd like

Make promise creation fail on conflict. This will require a small refactor given that we still need to ignore creation conflicts when dealing with promises created by schedules.

Additional context

We found this behaviour when doing Callback+Promise creation. Given that this operation doesn't need to be atomic we split the two operations. However, we can do the same for Task+Promise creation given that this operation needs to be atomic.