fix(errors): properly assign APIError.body (#219)

Author: stainless-bot

Diff for: src/finch/_exceptions.py

@@ -41,6 +41,7 @@ def __init__(self, message: str, request: httpx.Request, *, body: object | None)
         super().__init__(message)
         self.request = request
         self.message = message
+        self.body = body
 
 
 class APIResponseValidationError(APIError):