Author: Rushikesh
MDL-33663 Grading methods: Appropriate error message for negative grades
http://git.moodle.org/gw?p=moodle.git;a=commit;h=ef8dc8570b9114101d5ac78161ec46f148973d7b
http://github.com/moodle/moodle/commit/ef8dc8570b9114101d5ac78161ec46f148973d7b
+ 3.1 en [err_maxscoreisnegative,gradingform_guide]
+ 3.1 en [err_scoreinvalid,gradingform_guide]
+ 3.1 en [err_scoreisnegative,gradingform_guide]