refactoring of standalone epub dumper, and other things

- extracted abstract class EpubDumperCallback that handles saving data into a epublib Book object
- removed saving of cleaned-up chapters into separate files
- fixed interruption of dumping on errors
small improvements

- remove unnecessary finals
- make EpubDumperCallback.log protected
- add logging when fetch fails
- extract PostNotFoundException to top level, make public
remove incorrect throws declaration
make EpubDumperCallback public
actually make epubs on /dump request