Crash (NotADirectoryError / OSError) when the per-book output path is a file or symlink

[adisakshya]

Context

split_book.py, write_chunks.

Problem

Before writing, write_chunks unconditionally does:

if out_path.exists():
    shutil.rmtree(out_path)
out_path.mkdir(parents=True, exist_ok=True)

shutil.rmtree only works on plain directories. If <out>/<input-stem> already exists as something else, the call raises an uncaught exception and the tool dies with a raw Python traceback instead of the clear stderr messages used everywhere else in the tool (e.g. load_reader).

Reproduction (both confirmed)

Case A — plain file at the output path:

1. mkdir -p out_dir && touch out_dir/mybook (a file, not a directory, named after the input's stem)
2. python split_book.py mybook.pdf --out out_dir
3. Crashes with NotADirectoryError: [Errno 20] Not a directory: '.../out_dir/mybook'

Case B — symlink at the output path:

1. mkdir -p out_dir && ln -s /some/other/dir out_dir/mybook
2. python split_book.py mybook.pdf --out out_dir
3. Crashes with OSError: Cannot call rmtree on a symbolic link

Expected

A clear, actionable stderr message and a clean exit (code 1) — e.g. "Error: output path '...' exists and is not a plain directory; remove it or choose a different --out."

Actual

Raw, unhandled traceback in both cases (see above).

Acceptance criteria

• Both reproduction cases above produce a clear error message instead of a traceback.
• Regression tests cover both cases.

Note: the same shutil.rmtree call is also the source of a silent-data-loss bug (unrelated files in the output directory get deleted with no warning) — see the follow-up issue for that; a fix here should probably be designed together with it.