more detailed error handling in registry-api, show crates.io errors to user

[syphar]

Today I gave the registry api code a little love.

generally I wanted:

• retry only when it's worth retrying (= on 5xx & connection errors)
• parse the crates.io api error responses, and show the message to the user. For now I assume that 4xx responses are json / can be parsed, and 5xx are plain text.
• use the status code we get from crates.io as our own status code.
• use a enum as error type, so we can use the specific error information in the search-handler.
• to move some tests from integration-like (where we tested crates.io api things via the search-handler in web::handlers::releases), to more unit-like tests directly on the RegistryApi struct.

In the code I discovered some code that tried to parse error messages out out successful responses. I assume that this is legacy, and dropped it. [To be safe, I also asked](#t-crates-io > api , error responses).

( I didn't put any time in design for the error message, feel free to improve)

[GuillaumeGomez]

Thanks!