Changed HelpArgumentAction to throw exception rather than exit

- As it stood, HelpArgumentAction.run() called System.exit, regardless of
  if it was triggered by a call to parseArgs() or parseArgsOrExit()  The
  problem with this is that the call to System.exit() makes testing any
  code which triggers a call to HelpArgumentAction very difficult.  With
  this change, now parseArgs() raises a HelpScreenException when --help
  appears as an argument, and the caller can handle this as they see fit.
  parseArgsOrExit() exits the program if --help is supplied as an argument
  as was the case before.