e36c1c2aa1
Gets rid of synthetic_error, and makes the various send_* methods return `Result<Response, Error>`. Introduces a new error type "HTTP", which represents an error due to status codes 4xx or 5xx. The HTTP error type contains a boxed Response, so users can read the actual response if they want. Adds an `error_for_status` setting to disable the functionality of treating 4xx and 5xx as errors. Adds .unwrap() to a lot of tests. Fixes #128.