--ignore-existing does not meet semantics of scheduler pools (and accounts too)

[zlobober]

If there is a pool bar/foo, and you are trying to create a pool baz/foo, you get an expected error:

$ yt create scheduler_pool --attributes '{name=foo; parent_name=baz; pool_tree=default}'
Failed to attach child "foo" to scheduler pool tree "default"    
    origin          precision on 2024-04-17T12:43:22.003595Z
Pool tree "default" already contains pool with name "foo"    
    code            501    
    origin          precision on 2024-04-17T12:43:22.003595Z

(Recall that it is not allowed to have two pools with the same name at different places of the pool tree).

You try to prevent this error from happening by adding an --ignore-existing flag, but the error still happens; probably, because the ignore-existing checks whether there is a pool in exactly the same location; while the scheduler semantics of "exists" spans across the whole pool tree.

Let's improve the behavior for --ignore-existing by allowing it to succeed if there is already a pool in the different part of a tree.