descale: Return more useful input < output wh err

This suffers from the same issue old VS exceptions suffered from, in that it isn't clear. This should ideally be changed in the plugin itself, and once that's been done, I will remove it here.

vskernels/kernels/custom.py

@@ -1,5 +1,5 @@
 from __future__ import annotations
-from stgpytools import KwargsT, inject_self
+from stgpytools import CustomValueError, KwargsT, inject_self
 from inspect import Signature
 
 from vstools import vs, core
@@ -62,7 +62,16 @@ def descale_function(  # type: ignore[override]
             if k not in Signature.from_callable(self._modify_kernel_func).parameters.keys()
         }
 
-        return core.descale.Decustom(clip, width, height, kernel, int(support), *args, **clean_kwargs)
+        try:
+            return core.descale.Decustom(clip, width, height, kernel, int(support), *args, **clean_kwargs)
+        except vs.Error as e:
+            if 'Output dimension must be' in str(e):
+                raise CustomValueError(
+                    f'Output dimension ({width}x{height}) must be less than or equal to '
+                    f'input dimension ({clip.width}x{clip.height}).', self.__class__
+                )
+
+            raise CustomValueError(e, self.__class__)
 
     def get_params_args(
         self, is_descale: bool, clip: vs.VideoNode, width: int | None = None, height: int | None = None, **kwargs: Any