Change error to debug

IMO this is a better workaround than using the old-style
error-pattern: syntax :-)

Author: catamorphism

src/test/compile-fail/index_message.rs

@@ -1,5 +1,4 @@
-// error-pattern:cannot index a value of type `()`
 fn main() {
     let z = ();
-    log(error, z[0]);
+    log(debug, z[0]); //! ERROR cannot index a value of type `()`
 }