`mkdir` doesn't error on already-existing directory

Bug report form

• I have done a basic search of the issue tracker to find any existing issues that are similar.
• I have checked that my version is at least the latest stable release available via my installation method.

Describe the bug

mkdir for an existing path, including a path created by mkdir itself, returns exit code 0 as if a directory was created. worse, mkdir --verbose reports the path as created: true.

to be clear, this doesn't match the behavior of GNU/coreutils mkdir or uutils/coreutils mkdir

How to reproduce

> cd (mktemp --directory)
> mkdir foo
> $ans.exit_code
0
> mkdir foo
> $ans.exit_code
0
> mkdir --verbose foo
╭───┬─────────────────────────────────────────────────────────────────────┬─────────┬───────╮
│ # │                                path                                 │ created │ error │
├───┼─────────────────────────────────────────────────────────────────────┼─────────┼───────┤
│ 0 │ /var/folders/n1/2s4wgvw17z75hspycqvqh8k00000gn/T/tmp.F2LC1Klmii/foo │ true    │       │
╰───┴─────────────────────────────────────────────────────────────────────┴─────────┴───────╯
> ^/opt/homebrew/opt/uutils-coreutils/libexec/uubin/mkdir foo
mkdir: foo: File exists

Expected behavior

error on the existing path. exit code should be 1 if at least one path was not created because it already exists (even though other paths may have been created). this matches uutils

also, mkdir --verbose should show it as an error.

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