Graph Slider Ops: Show error when no valid keys are found

When using graph slider operators like D9374
it showed a warning when no keys were selected.
However since that stops the modal operation it should be an Error.
Also the message was misleading
since it could error for different reasons than stated.

Reviewed by: Sybren A. Stüvel
Differential Revision: https://developer.blender.org/D13655
Ref: D13655

source/blender/editors/space_graph/graph_slider_ops.c

@@ -313,8 +313,7 @@ static int graph_slider_invoke(bContext *C, wmOperator *op, const wmEvent *event
   ED_slider_init(gso->slider, event);
 
   if (gso->bezt_arr_list.first == NULL) {
-    WM_report(RPT_WARNING,
-              "Fcurve Slider: Can't work on baked channels. Unbake them and try again.");
+    WM_report(RPT_ERROR, "Cannot find keys to operate on.");
     graph_slider_exit(C, op);
     return OPERATOR_CANCELLED;
   }