Don't try to sort hashable, map to string (pydata#8172)

* Update dataset.py

* Update dataset.py

* Update dataset.py

* Remove sorted, keep it simple.

* Update dataset.py

* tests fail if tweaking this too much, just map to str then.

xarray/core/dataset.py

@@ -5284,7 +5284,7 @@ def to_stacked_array(
             if missing_sample_dims:
                 raise ValueError(
                     "Variables in the dataset must contain all ``sample_dims`` "
-                    f"({sample_dims!r}) but '{key}' misses {sorted(missing_sample_dims)}"
+                    f"({sample_dims!r}) but '{key}' misses {sorted(map(str, missing_sample_dims))}"
                 )
 
         def stack_dataarray(da):