re-export client types and `isahc::Error`

As it stands, `Client`, `Client` and friends are not nameable by consumers of the crate, which causes issues if the user wants to put a client in a struct.

Additionally, `IsahcClient` wraps `isahc::Error` into `Exception`, so if you want to downcast you need to add the right version of `isahc` as a dependency: https://www.reddit.com/r/rust/comments/dy273n/hey_rustaceans_got_an_easy_question_ask_here/f8badd4/

(I'm posting this before that user has replied so I'm not 100% sure if that's the problem they're running into.)