Establish `ConnectorError` for errors from SmithyConnector (#744)

* Introduce ClientError to differentiate between different error types coming from HTTP connectors

* Add test

* Fix tests

* Fix the DVR connections

* Fix event-stream client

* clippy cleanups

* Fix poorly named variables in downcast error

* Update docs

* Rename ClientError to ConnectorError

* Fix more incorrectly named errors

* Update rust-runtime/smithy-http/src/result.rs

Co-authored-by: John DiSanti <jdisanti@amazon.com>

* Update changelog

* Update SDK_CHANGELOG.md

Co-authored-by: John DiSanti <jdisanti@amazon.com>