kickstart should print a more helpful error message if the executable is missing

If the user's executable is missing or otherwise unable to run, kickstart prints the error message

kickstart[4156952]: pegasus-kickstart.c[673]: FATAL: The main job specification is invalid or missing.

This is not helpful to the non-expert user. Something more human-readable like

"The executable /path/to/program cannot be found, please check that the executable is available at URL"

would be more helpful.