Manually implement backtrace for custom error type

[scottopell]

What does this PR do?

Motivation

Before:

Error: SerdeYaml(Error("generator[0].unix_datagram.variant: invalid type: enum, expected any value", line: 7, column: 16))

After:

Error: WithBacktrace { inner: Wrapped(Error("generator[0].unix_datagram.variant: invalid type: enum, expected any value", line: 7, column: 16)), backtrace: Backtrace [{ fn: "lading::Error::bt", file: "./home/ubuntu/dev/lading/lading/src/bin/lading.rs", line: 79 }, { fn: "lading::Error::wrap", file: "./home/ubuntu/dev/lading/lading/src/bin/lading.rs", line: 74 }, { fn: "core::ops::function::FnOnce::call_once", file: "./rustc/82e1608dfa6e0b5569232559e3d385fea5a93112/library/core/src/ops/function.rs", line: 250 }, { fn: "core::result::Result<T,E>::map_err", file: "./rustc/82e1608dfa6e0b5569232559e3d385fea5a93112/library/core/src/result.rs", line: 829 }, { fn: "lading::get_config", file: "./home/ubuntu/dev/lading/lading/src/bin/lading.rs", line: 268 }, { fn: "lading::main", file: "./home/ubuntu/dev/lading/lading/src/bin/lading.rs", line: 584 }, { fn: "core::ops::function::FnOnce::call_once", file: "./rustc/82e1608dfa6e0b5569232559e3d385fea5a93112/library/core/src/ops/function.rs", line: 250 }, { fn: "std::sys_common::backtrace::__rust_begin_short_backtrace", file: "./rustc/82e1608dfa6e0b5569232559e3d385fea5a93112/library/std/src/sys_common/backtrace.rs", line: 154 }, { fn: "std::rt::lang_start::{{closure}}", file: "./rustc/82e1608dfa6e0b5569232559e3d385fea5a93112/library/std/src/rt.rs", line: 167 }] }

Related issues

A list of issues either fixed, containing architectural discussions, otherwise relevant
for this Pull Request.

Additional Notes

Anything else we should know when reviewing?

[scottopell]

inspired by dtolnay/thiserror#257 (comment)

[scottopell]

but none of this is getting merged, so closing for now.