Add RootMessage function

[Bigyin1]

RootMessage returns message associated with root error in stack. This allows us to get root error message and use it to, i.e, return to user without any additional structs.

[palantirtech]

Thanks for your interest in palantir/stacktrace, @Bigyin1! Before we can accept your pull request, you need to sign our contributor license agreement - just visit https://cla.palantir.com/ and follow the instructions. Once you sign, I'll automatically update this pull request.