Summary:
Calling `Exn.backtrace` doesn't give us the current backtrace, only the one of
the latest exception raised. Change the logic of toplevel exception catching to
print the right backtrace.
Also, do not use `Format` to print from `Config` as `Logging` may have messed
with it already.
Finally, throw in some colours!
Reviewed By: jberdine
Differential Revision: D5764825
fbshipit-source-id: cd51688
Jules Villard