Add backtrace to azure_core::Error

We should add a [`Backtrace`](https://doc.rust-lang.org/stable/std/backtrace/struct.Backtrace.html) to `azure_core::Error` so that, when `RUST_BACKTRACE=1` (or "all", whatever) is passed developers can get a full backtrace on `Error`s we return.

We'd call `Backtrace::capture()` in our various construction functions.