Add general-purpose datastructure for marshalling exceptions
Summary: Ocaml doesn't allow us to pattern-match on marshalled exceptions: only obtain its string message. (The compiler and typesystem allow it, but the documentation says "don't\!"). I'm adding a new datatype to carry around marshalled exceptions in. Putting it here in marshal_tools because everyone who amrshals might want to use it.
Reviewed By: arxanas
Differential Revision:
D5155539
fbshipit-source-id:
a0da1cac53ace7bf9f11c9eaad5f68306b39f2f1