fix: throw an error if the status code is a faulty one and stop the stream if a 204 is received

[dunglas]

Partial fix for #7.

A better fix would be to only allow 200, as stated by the spec, but this isn't possible without a BC break: it's not possible to create a new reqwest::Error for the other status codes.
We have a similar problem with the Content-Type: it must be set by the server to text/event-stream, and we should throw an error if it's not the case, but there is currently no corresponding reqwest::Error.

A better solution (at the cost of a BC break), is to introduce our custom error type (using thiserror or a similar crate). I suggest merging this patch as a quick fix for the current version, and to start working on a better implem in a new major version.

[jpopesculian]

Hi @dunglas! Thank you again for helping me bring things up to spec! I decided to go with latter approach you suggested with #9 which hopefully brings this a lot closer to what it should be. Take a look and let me know what you think!