fix: keep postponing when a task cannot be saved - #87

fix: keep postponing when a task cannot be saved

During panic dismissal a "423 Locked" from Xandikos (its git index
was locked by another write) aborted the whole run with a traceback.
A PutError on saving a postponed task is now reported, and the rest of
the list is still postponed.  The task is not marked as postponed.  If
the parent a child needs moved cannot be saved, the child is reported
as not postponed.  A parent saved before its child fails stays moved.

Prompt: (traceback from `plann interactive manage-tasks` pasted, ending in "caldav.lib.error.PutError: PutError at '423 Locked'" from _procrastinate's x.save())
Followup-Prompt: I think we have a github issue and some design document on "retry logic in the caldav library".  Please find and add the 423-retry into the same design document.  Fix the try-and-continue in plann. [the design document is in the caldav repository]

Assisted-By: Claude Opus 5.5 <noreply@anthropic.com>
Reviewed-by: Tobias Brox <tobias@redpill-linpro.com>