Avoid crash if not provide book_name

[xyb]

Instead of showing an ugly crash message like the one below to the user when they don't provide any arguments, it would be better to display a more user-friendly message:

❯ bbook_maker
Traceback (most recent call last):
  File "/Users/xyb/.virtualenvs/bilingual_book_maker/bin/bbook_maker", line 8, in <module>
    sys.exit(main())
             ^^^^^^
  File "/Users/xyb/projects/bilingual_book_maker/book_maker/cli.py", line 328, in main
    if not os.path.isfile(options.book_name):
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "<frozen genericpath>", line 30, in isfile
TypeError: stat: path should be string, bytes, os.PathLike or integer, not NoneType

A more appropriate message would be:

❯ bbook_maker
Error: please provide the path of your book using --book_name <path>

[yihong0618]

Nice catch thanks