gh-121018: Fix more cases of exiting in argparse when exit_on_error=False (GH-121056)

* parse_intermixed_args() now raises ArgumentError instead of calling
  error() if exit_on_error is false.
* Internal code now always raises ArgumentError instead of calling
  error(). It is then caught at the higher level and error() is called if
  exit_on_error is true.