feat: add error if env var is missing

app.py

@@ -7,6 +7,9 @@ load_dotenv()
 
 token = os.environ.get("HF_TOKEN")
 
+if not token:
+    raise gr.Error("HF_TOKEN is not set.")
+
 """
 For more information on `huggingface_hub` Inference API support, please check the docs: https://huggingface.co/docs/huggingface_hub/v0.22.2/en/guides/inference
 """