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.
This loads a list of top domain names (e.g. from
https://tranco-list.eu/) and tries to fetch them all, in parallel. This
can be used to exercise ureq and find panics.
