add input validation for set_modification_count

update error macros for set_modification_count

scene/resources/skeleton_modification_stack_2d.cpp

@@ -195,6 +195,7 @@ void SkeletonModificationStack2D::set_modification(int p_mod_idx, Ref<SkeletonMo
 }
 
 void SkeletonModificationStack2D::set_modification_count(int p_count) {
+	ERR_FAIL_COND_MSG(p_count < 0, "Modification count cannot be less than zero.");
 	modifications.resize(p_count);
 	notify_property_list_changed();
 

scene/resources/skeleton_modification_stack_3d.cpp

@@ -149,6 +149,7 @@ void SkeletonModificationStack3D::set_modification(int p_mod_idx, Ref<SkeletonMo
 }
 
 void SkeletonModificationStack3D::set_modification_count(int p_count) {
+	ERR_FAIL_COND_MSG(p_count < 0, "Modification count cannot be less than zero.");
 	modifications.resize(p_count);
 	notify_property_list_changed();
 }