feat!: wrap Client::new() return in a Result

BREAKING CHANGE: `Client::new()` now returns a `Result`.

src/v1/error.rs

@@ -12,7 +12,7 @@ impl fmt::Display for ApiError {
 }
 impl Error for ApiError {}
 
-#[derive(Debug, thiserror::Error)]
+#[derive(Debug, PartialEq, thiserror::Error)]
 pub enum ClientError {
     #[error("You must either set the `MISTRAL_API_KEY` environment variable or specify it in `Client::new(api_key, ...).")]
     ApiKeyError,