Use the anyhow crate for error handling

[martinling]

Use the anyhow crate for error handling.

This crate provides:

• The anyhow::Error type, which wraps any other error type, and can thus replace the whole error type hierarchy we have currently including PacketryError, CaptureError, StreamError, ModelError etc.

• A bail! macro which can simplify sites where we just want to return an error with some text.

• A Context trait, implemented for Option and Result, which allows easily attaching context information when turning an existing None or Err value into an anyhow::Error. This neatly replaces some existing patterns and our OnBug trait.

As well as simplifying the code, this change enables us to display a backtrace for any error - this is implemented in the second commit. Backtraces are only captured if RUST_BACKTRACE or RUST_LIB_BACKTRACE are set.

[miek]

I've pushed some CI fixes, could you rebase?

[martinling]

Passing CI now.