Author: Paul Holden
MDL-79307 assignfeedback_editpdf: more informative task failure.
If ghostscript is not correctly configured the task will never
succeed, so error out earlier with a more informative exception
explaining this.
http://git.moodle.org/gw?p=moodle.git;a=commit;h=f4b502a4935b1cdcf38e10888f71f342ac849695
http://github.com/moodle/moodle/commit/f4b502a4935b1cdcf38e10888f71f342ac849695
+ 4.2 en [pathtogserror,assignfeedback_editpdf]