Keep new and removed state of non-working days on error state

[oliverguenther]

When the non working days cannot be saved for some reason, keep them around so the user can either correct their mistake, or wait for the changes to be applied.

[github-actions]

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[cbliard]

Works well. Thanks.

The only thing that is not preserved is when one wants to rename a day, they can't do it easily: they have to remove the day and add it again with another name. When doing so and the cahnge cannot be applied, then the day is lost: it only keeps the removal and not the addition, but that's really an edge case.

[oliverguenther]

When doing so and the cahnge cannot be applied, then the day is lost: it only keeps the removal and not the addition, but that's really an edge case.

Good point, but I think we can ignore that edge case in favor of a better blocking when an active job is processing