direct export a selection as saved by Bookcreator plugin

[Klap-in]

Hi!
An feature request for bookcreator Klap-in/dokuwiki-plugin-bookcreator#97 is implemented with some changes to dw2pdf. Could you add these changes to the ODT plugin as well?

splitbrain/dokuwiki-plugin-dw2pdf#422
the changes are:

• use exceptions for non-normal cases (the functions from bookcreator throw exceptions, so this made them easier to catch)
• use functions from bookcreator to get the list of pages and title.

Not sure if all relevant changes of dw2pdf are in ODT, but probably this helps to compare. See the 'action.php' in this list:
See also splitbrain/dokuwiki-plugin-dw2pdf@2d9cd42...master

Eventually, I could create a PR from this if you prefer, but that may take some time.