[booktopipeline] Add a check to disallow adding a book that already exists

This is important as if a book is added which has already been done,
then an analyse job will be added every time a page is OCRed, which
will clog up the pipeline with unnecessary work. Also if a book was
added with the same name but differently named files, or a different
number of pages, the results would almost certainly not be as
intended.

In the case of a book really wanting to be added with a particular
name, either the original directory can be removed on S3, or "v2"
or similar can be appended to the book name before calling
booktopipeline.

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