lib/classes/PluginManager.js/PluginManager/getCommand
class PluginManager:  getCommand(commandsArray) {
    return _.reduce(commandsArray, (current, name, index) => {
      if (name in current.commands) {
        return current.commands[name];
      }
      const commandName = commandsArray.slice(0, index + 1).join(' ');
      const errorMessage = [
        `Command "${commandName}" not found`,
        ' Run "serverless help" for a list of all available commands.',
      ].join();
      throw new this.serverless.classes.Error(errorMessage);
    }, { commands: this.commands });
  }