investigate a way to get backtraces independent of RUST_BACKTRACE

[dherman]

Backtraces would be useful for verbose diagnostics, not just for Volta maintainers. But we don't want to require Volta users to know about the RUST_BACKTRACE environment variable (and it could cause nasty interactions between Volta and other software written in Rust). Investigate if there are mechanisms for getting backtraces in the failure library (or a fork of failure) without being sensitive to that environment variable.

[chriskrycho]

Since we're using thiserror, it should be straightforward to get backtraces throughout the application, and this does not require using RUST_BACKTRACE. We can then use thiserror's support for std::backtrace::Backtrace to make it available in our error reporting, and then perhaps couple it to --verbose reporting etc.