Raise ValueError in _get_ordered_vertices() with "mixed" order (#595)

* Raise a ValueError in `_get_ordered_vertices()` with `"mixed"` core dim order

* Replace ValueError with NotImplementedError

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Pascal Bourgault <[email protected]>

Author: 3 people

cf_xarray/helpers.py

@@ -338,6 +338,12 @@ def _get_ordered_vertices(
         elif order == "descending":
             endpoints = np.maximum(bounds[..., :, 0], bounds[..., :, 1])
             last_endpoint = np.minimum(bounds[..., -1, 0], bounds[..., -1, 1])
+        else:
+            raise NotImplementedError(
+                f"Cannot determine vertices for non-monotonic bounds with {order} core "
+                "dimension orders. Try normalizing the coordinates to a monotonic "
+                "convention and try again."
+            )
 
         vertices = np.concatenate(
             [endpoints, np.expand_dims(last_endpoint, axis=-1)], axis=-1