Do not signal error on successful non-`200` HTTP status codes

Currently, `plz.el` signals `plz-http-error` on anything besides a status 200 code. According to [Mozilla](https://developer.mozilla.org/en-US/docs/Web/HTTP/Status), only status codes between 400-599 are errors.

I was confused when I received a successful [201 status code](https://developer.mozilla.org/en-US/docs/Web/HTTP/Status/201) and `plz.el` signaled an HTTP error. 

