That the file existed said only that the server sent a body, and an Audible error is
a 200 with a JSON body like any other response. Dinah's downloader renames by
Content-Disposition, so such a body landed in the book's folder under whatever
Audible called it and the title was recorded as having its PDF.
Check the payload: a file named .pdf must carry the PDF header, and nothing may begin
with the opening character of a JSON or markup document. A rejected download is
deleted rather than left in the library, which also lets the empty-folder cleanup
run, and the file is added to the path cache only once it has passed.
Co-authored-by: rmcrackan <rmcrackan@gmail.com>
