Fix fix for 8652 (failures with bad XML metadata for DjVu files)

Fix fix for 8652 (failures with bad XML metadata for DjVu files)
The mid-level functions in Image now fail gracefully on bogus data, as happens when the djvutoxml executable is missing.

Removed the bogus exception check in ImagePage high-level code (which actually just causes a fatal error by calling a nonexistent method :)

Also changed Image::initializeMultiPageXML to do clean lazy loading as Image::load does, and to return true/false as to success/failure for cleaner code using it.