Author: Ankit Agarwal
MDL-33646 book: Adding a notice instead of error when there is no content in the book
http://git.moodle.org/gw?p=moodle.git;a=commit;h=5747c6bccd134e9982f15e4e1e5d0fa11f02ba06
http://github.com/moodle/moodle/commit/5747c6bccd134e9982f15e4e1e5d0fa11f02ba06
+ 2.3 en [nocontent,mod_book]