add assert path.exists() for readable error

tests/conftest.py

@@ -1,7 +1,10 @@
 import pytest
+from pathlib import Path
 from microwink import SegModel
 
 
 @pytest.fixture
 def seg_model() -> SegModel:
-    return SegModel.from_path("./models/seg_model.onnx")
+    path = Path("./models/seg_model.onnx")
+    assert path.exists()
+    return SegModel.from_path(path)