Allow capturing Backtraces from anyhow::Error

[hgmich]

With dtolnay/anyhow#143 merged as 1.0.39, anyhow now supports backtrace capturing on stable Rust through an optional backtrace feature which uses the backtrace crate (as already used by sentry-backtrace). It would be useful to capture this information for anyhow errors to provide a replacement for the functionality that previously existed in sentry-failure.