Add OxiAPIError exception class for improved error handling

- Introduced a new `OxiAPIError` class to standardize error reporting in the OxiAPI.
- The class includes an optional status code for enhanced context in error messages.

oxi/exception.py

@@ -0,0 +1,12 @@
+from typing import Optional
+
+
+class OxiAPIError(Exception):
+    def __init__(self, message: str, status_code: Optional[int] = None):
+        super().__init__(message)
+        self.status_code = status_code
+
+    def __str__(self):
+        if self.status_code is not None:
+            return f"OxiAPIError: {self.args[0]} (HTTP {self.status_code})"
+        return f"OxiAPIError: {self.args[0]}"