_checkForMissingExecutable(executableFile, executableDir, subcommandName) {
    if (fs.existsSync(executableFile)) return;

    const executableDirMessage = executableDir
      ? `searched for local subcommand relative to directory '${executableDir}'`
      : 'no directory for search for local subcommand, use .executableDir() to supply a custom directory';
    const executableMissing = `'${executableFile}' does not exist
 - if '${subcommandName}' is not meant to be an executable command, remove description parameter from '.command()' and use '.description()' instead
 - if the default executable name is not suitable, use the executableFile option to supply a custom name or path
 - ${executableDirMessage}`;
    throw new Error(executableMissing);
  }
Throw if expected executable is missing. Add lots of help for author.

@param {string} executableFile
@param {string} executableDir
@param {string} subcommandName