map_over_datasets: fix error message for wrong result type (#10016)

* `map_over_datasets`: fix error message for wrong result type

* newline for result

Author: mathause

xarray/core/datatree_mapping.py

@@ -160,8 +160,8 @@ def _check_single_set_return_values(path_to_node: str, obj: Any) -> int | None:
         isinstance(r, Dataset | None) for r in obj
     ):
         raise TypeError(
-            f"the result of calling func on the node at position is not a Dataset or None "
-            f"or a tuple of such types: {obj!r}"
+            f"the result of calling func on the node at position '{path_to_node}' is"
+            f" not a Dataset or None or a tuple of such types:\n{obj!r}"
         )
 
     return len(obj)

xarray/tests/test_datatree_mapping.py

@@ -72,7 +72,7 @@ def test_return_wrong_type(self, simple_datatree):
         with pytest.raises(
             TypeError,
             match=re.escape(
-                "the result of calling func on the node at position is not a "
+                "the result of calling func on the node at position '.' is not a "
                 "Dataset or None or a tuple of such types"
             ),
         ):
@@ -84,7 +84,7 @@ def test_return_tuple_of_wrong_types(self, simple_datatree):
         with pytest.raises(
             TypeError,
             match=re.escape(
-                "the result of calling func on the node at position is not a "
+                "the result of calling func on the node at position '.' is not a "
                 "Dataset or None or a tuple of such types"
             ),
         ):