Also correct the default from `LETTER` to `A4`, since the
label sheets defined so far are actually `A4`.
Co-authored-by: gitolicious <26963495+gitolicious@users.noreply.github.com>
- don't fail, when the directory doesn't exist but create it instead
- handle failures to create the output directory properly
- use f-string formatting to simplify building the output filename
- use OS-independent `os.path.join` to build the output path
