Return ExitError type (#196)

Shove the original command error in the error tree.
This allows checking for the specific exit code by code like this:

	var exitError *exec.ExitError
	if errors.As(err, &exitError) {

Added command.FormattedError type to work around issue of not
returning the original exec.ExitError type. The Unwrap() method returns
the original error, so the above example will now work.

Added UpdateRef (#193)

Update internal_logger.go (#185)