pull request: add exception handling in nodes.rb

[zhiyuchen]

hi Bruce,

I found your jsonpath lib a very useful tool to parse json data in ruby, thanks so much for sharing!

I'm recently using this in my current project and run into a couple issues, so I would like to contribute to this repo if possible.

Could you please review the change that I made and pull the change to the main repo if that looks good to you?

Thank you a lot!
Zhiyu