Do not print backtraces on program interrupt

[NathanReb]

As pointed out in #216, it seems that in some cases opam-monorepo prints an "uncaught exception" message, along with a backtrace when the user sends an interrupt (SIGINT via CTRC-C for instance).

It'd be great if it exited more gracefully in this case.

@samoht mentioned that he uses CTRL-C to skip prompt, i.e. as a way to answer no in our case. We might want to look into this as well but it seems we need to properly handle interruption/termination signals globally first.