Author: Frederic Massart
MDL-51504 tool_lp: Validate the templates' due date
http://git.moodle.org/gw?p=moodle.git;a=commit;h=0c65d52c7f9ae737ca1acf94fa66f1a49b77d595
http://github.com/moodle/moodle/commit/0c65d52c7f9ae737ca1acf94fa66f1a49b77d595
+ 3.1 en [errorcannotsetduedateinthepast,tool_lp]
+ 3.1 en [errorcannotchangeapastduedate,tool_lp]