Fix crash on older boost when receiving an exception from an MThread

Fix crash on older boost when receiving an exception from an MThread

for older boost fcontext versions, we would return a boolean that said 'we
caught an exception for you and stored it in ctx', but we would not actually
retrieve the origin ctx, and then blindly attempt to rethrow the exception
(not) stored in the ctx we did have, leading to a crash. We now send back the
actual ctx, and check it for a stored exception.