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=5219cc87ec7d398d0130c308d852d0c811425c22
http://github.com/moodle/moodle/commit/5219cc87ec7d398d0130c308d852d0c811425c22
+ 2.4 en [nocontent,mod_book]