Expose UnrecoverableError

This is more of a small feature request than a bug report. It'd be great to have a way of knowing if an error thrown by the library is an instance of `UnrecoverableError`, which is private. This could be easily done by either exporting the class or better yet, providing a function that given an error, checks internally for you (so consumers of the library can't create those errors themselves).

Right now, the solution is to stringify the error and check the string, which isn't super robust.

If you think this makes sense, I can create a PR myself! Please, let me know.