feat: add NotImplementedError error for index changing methods Dask (#744)

* add notImplemented error for index changing methods Dask

* add NotImplementedError to index changing methods Dask

* add NotImplementedError in Dask drop_null/count

* Update expr.py

---------

Co-authored-by: Marco Edward Gorelli <marcogorelli@protonmail.com>

Author: anopsy

narwhals/_dask/expr.py

@@ -4,6 +4,7 @@
 from typing import TYPE_CHECKING
 from typing import Any
 from typing import Callable
+from typing import NoReturn
 
 from narwhals.dependencies import get_dask
 
@@ -420,6 +421,16 @@ def round(self, decimals: int) -> Self:
             returns_scalar=False,
         )
 
+    def drop_nulls(self) -> NoReturn:
+        # We can't (yet?) allow methods which modify the index
+        msg = "`Expr.drop_nulls` is not supported for the Dask backend. Please use `LazyFrame.drop_nulls` instead."
+        raise NotImplementedError(msg)
+
+    def sort(self, *, descending: bool = False, nulls_last: bool = False) -> NoReturn:
+        # We can't (yet?) allow methods which modify the index
+        msg = "`Expr.sort` is not supported for the Dask backend. Please use `LazyFrame.sort` instead."
+        raise NotImplementedError(msg)
+
     def abs(self) -> Self:
         return self._from_call(
             lambda _input: _input.abs(),