Summary:
If the waited upon object throws an exception when blockingWait is called within a fiber context this can cause a SIGABT as it is not permissible to yield (due to the inner baton) in an exception context.

To fix this, instead capture the exception and permit destruction to occur outside of exception handling.

Reviewed By: andriigrynenko

Differential Revision: D19951124

fbshipit-source-id: cf1bf9a62557147863390132f551d08e20c02c2d