Check engine http response status

[Mark Phelps]

We arent currently checking the response status of the call to get evaluation data:

flipt-client-sdks/flipt-engine/src/parser/mod.rs

Line 59 in 3792aa3

 Ok(resp) => resp, 

It seems the reqwest lib we are using does not return errors for most non 2xx responses by default: https://stackoverflow.com/questions/73999866/rust-response-error-handling-using-reqwest#comment130659291_73999866

We may need to switch to error_for_status and handle appropriately