Do not capture backtrace without RUST_BACKTRACE=1

[pravic]

Subj.

The problem described here: rust-lang-nursery/rustup.rs#591. Guess issue related to this crate since backtrace-rs is explicit thing to deal with it.

[brson]

Thanks for filing this. I'm amenable to making this behavior an option at least, but I also want to figure out why you are seeing so much disk thrashing on windows - it shouldn't be that expensive to capture a backtrace. Many languages capture them for every exception, and that's a model I want to support, just having backtraces always available.

[pravic]

I'll answer in that issue better.

[pravic]

Many languages capture them for every exception, and that's a model I want to support, just having backtraces always available.

We can use RtlCaptureStackBackTrace, it does not require debug symbols to work.

[brson]

Let's just go ahead and do this. No backtraces unless RUST_BACKTRACE is set and not equal to "0". The backtrace field will need to become an option.