backtrace: add backtrace of this

[yorkie]

Hi, rust core team,

task '<main>' failed at 'called `Option::unwrap()` on a `None` value', /Users/rustbuild/src/rust-buildbot/slave/nightly-mac/build/src/libcore/option.rs:265

I have occurred this, how hard to debug, right?
IMO if we provide a detailed error info(like v8) would be better for developers :p

[huonw]

If you run with the RUST_BACKTRACE=1 environment variable you can get a printed backtrace. You can also break on rust_fail in your favourite debugger.

(NB. one should try to avoid calling .unwrap ever, propagating errors is preferable, or even just using foo.expect("some useful message") is better.)

[yorkie]

Ok, got it, you means if someone wanna debug the program, it is better to do set that env-var, am i right?

[yorkie]

closing this first, thank you :)