Applescript exits with error when run via launchd
As bjbk commented, the problem was that osascript did not have Assistive Access. While it is easy to add .app programs to this list (System Preferences > Security &amp; Privacy > Accessibility), OS X doesn't allow binaries to be added through the GUI. See Jacob Palmela's blog for how to add a program via the command line, or just copy and paste the following command, substituting in the path of the program to be added where indicated before executing: 

sudo sqlite3 /Library/Application\ Support/com.apple.TCC/TCC.db "INSERT or REPLACE INTO access VALUES('kTCCServiceAccessibility','PROGRAM-TO-BE-ADDED',1,1,1,NULL)"


sudo is necessary because the sqlite3 db file is owned by root.