refactor: split client and server errors (#1122)

* split client and server error

* cargo fmt

* remove old code

* fix tests

* cargo fmt

* fix rustdoc links

* make clippy happy

* Update server/src/future.rs

* Update client/http-client/src/client.rs

* Update client/http-client/src/client.rs

* Update core/src/client/async_client/helpers.rs

* Update core/src/client/async_client/helpers.rs

* Update core/src/client/async_client/mod.rs

* Update core/src/client/async_client/mod.rs

* fix more todos

* unused dep: soketto

* remove unused error variant

* Update core/src/lib.rs

* cargo fmt

* fix grumbles: move client error to own mod

* simplify imports

* Update core/src/client/error.rs