make const_err a hard error

src/test/ui/consts/const-eval/conditional_array_execution.rs

@@ -1,16 +1,8 @@
-// build-fail
-
-#![warn(const_err)]
-
 const X: u32 = 5;
 const Y: u32 = 6;
 const FOO: u32 = [X - Y, Y - X][(X < Y) as usize];
-//~^ WARN any use of this value will cause an error
-//~| WARN this was previously accepted by the compiler but is being phased out
+//~^ ERROR constant
 
 fn main() {
     println!("{}", FOO);
-    //~^ ERROR evaluation of constant value failed
-    //~| WARN erroneous constant used [const_err]
-    //~| WARN this was previously accepted by the compiler but is being phased out
 }