ml: disable not implemented k-fold validation in RTrees

modules/ml/src/precomp.hpp

@@ -151,6 +151,11 @@ namespace ml
                 CV_Error( CV_StsOutOfRange,
                           "params.CVFolds should be =0 (the tree is not pruned) "
                           "or n>0 (tree is pruned using n-fold cross-validation)" );
+            if(val > 1)
+                CV_Error( CV_StsNotImplemented,
+                          "tree pruning using cross-validation is not implemented."
+                          "Set CVFolds to 1");
+
             if( val == 1 )
                 val = 0;
             CVFolds = val;