rpclib: Improve error outputs.

This branch makes these changes to the way we display errors from arti-rpc-client-core:

1. We now obey the convention that an Error's Display implementation shouldn't display the error's source.
2. We use tor-error::Report in our ffi code so that error chain information is still available outside of Rust.
3. We provide a facility to expose the nonfatal errors that occur during a failed connect attempt.

Closes #1650 (closed).

Closes #1826 (closed).