Update error id to an unused one

Author: oli-obk

src/libsyntax/attr/builtin.rs

@@ -435,7 +435,7 @@ fn find_stability_generic<'a, I>(diagnostic: &Handler,
         if let Some(ref mut stab) = stab {
             stab.promotable = true;
         } else {
-            span_err!(diagnostic, item_sp, E0713,
+            span_err!(diagnostic, item_sp, E0717,
                       "rustc_promotable attribute must be paired with \
                        either stable or unstable attribute");
         }

src/libsyntax/diagnostic_list.rs

@@ -413,5 +413,5 @@ register_diagnostics! {
     E0694, // an unknown tool name found in scoped attributes
     E0703, // invalid ABI
     E0704, // incorrect visibility restriction
-    E0713, // rustc_promotable without stability attribute
+    E0717, // rustc_promotable without stability attribute
 }