Provide underlying error in ParseError

I would like to access the reason for the ParseError error, to provide better error message to the user.

If this is a feature you would consider to add, I can prepare a PR.

@greyblake @tobiasge is there any update on this? This is an important feature

I haven't touched it for a while. But thanks for raising the voice, I may do it if get some free time over the weekend...