Provide a way to instantiate ureq::Error::Transport for tests

[tgeoghegan]

I have some retry logic that retries requests on certain HTTP status codes and on ureq::Error::Transport. I want to test my logic that determines if an error is retryable by feeding it synthetic ureq::Error values. I can easily create an instance of ureq::Error::Status since ureq::Response::new is public (as noted in the doccomment, it is indeed useful for unit tests), but there's no way for a client to create a ureq::Error::Transport value.

[algesten]

Can you make an std::io::Error and use convert that with .into()?

[tgeoghegan]

It looks like that would let me construct the Transport variant of ureq::Error, which I think might suffice for the testing case I described. I'm afraid I've since deleted the tests in question, so I don't really remember. So unless you have someone else asking for this, or some other motivation to do it, feel free to close without action.

[algesten]

Thanks!