more friendly error message when renaming patterns

testsuite/compiler/renamer/varpatt.gf

@@ -0,0 +1,13 @@
+abstract varpatt = {
+
+-- this should raise error
+-- we cannot pattern match on functions
+
+cat D ;
+fun D1 : D ;
+    D2 : D ;
+
+fun d : D -> Int ;
+def d x = 1 ;
+
+}