main: Record and print full exception stack traces.

This turns on stack trace recording unconditionally (the same
effect as if OCAMLRUNPARAM=b was always supplied), and also prints
stack traces to the log file if there is an exception.

See also:
http://caml.inria.fr/pub/docs/manual-ocaml/libref/Printexc.html

Note that this requires OCaml >= 3.11.0.