Use of MRI-specific Exception hidden ivars does not work on JRuby or Rubinius

[headius]

You know the ones... "mesg" and "bt" or "backtrace". I assume you have to go at them directly because they're not settable in MRI, but neither JRuby nor Rubinius have those hidden vars. We need to add appropriate ways to reconstitute an Exception on JRuby and Rubinius.

There's also no tests for the behavior, so we have no promises to keep! Hooray!

[jemc]

In case you don't "know the ones":

https://github.com/tenderlove/psych/blob/5616a13f42a1a587670dfa9ed3d03648b7f41548/lib/psych/visitors/yaml_tree.rb#L204-L205
https://github.com/tenderlove/psych/blob/cb75738e594f9d7f7d4899e0f932fcb55a9ad1e1/ext/psych/psych_yaml_tree.c#L5-L13
https://github.com/tenderlove/psych/blob/7e7ccf6fa4ee084c673d5a52888e2e18a41bfb5b/ext/psych/psych_to_ruby.c#L9-L16