edit_document: join parent execution in synchronous/uninterruptible mode, to prevent spurious interrupts when cascaded executions run into each other;

back to more strict dependencies, even for canceled groups (reverting parts of 02936e77a07c); cancel passive tasks more actively via Exn.Interrupt, by treating them like ragular jobs here; attempts to re-assign canceled futures/promises raise Exn.Interrupt; tuned;

export Graph.get_entry for convenience;

eliminated old-style sys_error/SYS_ERROR in favour of exception Fail -- after careful checking that there is no overlap with existing handling of that; tuned some error messages;