Update E0391 to new format.

src/librustc_typeck/collect.rs

@@ -184,6 +184,7 @@ impl<'a,'tcx> CrateCtxt<'a,'tcx> {
 
         let mut err = struct_span_err!(tcx.sess, span, E0391,
             "unsupported cyclic reference between types/traits detected");
+        err.span_label(span, &format!("cyclic reference"));
 
         match cycle[0] {
             AstConvRequest::GetItemTypeScheme(def_id) |

src/test/compile-fail/issue-3907.rs

@@ -18,6 +18,7 @@ struct S {
 }
 
 impl Foo for S { //~ ERROR: `Foo` is not a trait
+                 //~| NOTE: not a trait
                  //~| NOTE: type aliases cannot be used for traits
     fn bar() { }
 }

src/test/compile-fail/issue-5035.rs

@@ -11,6 +11,7 @@
 trait I {}
 type K = I;
 impl K for isize {} //~ ERROR: `K` is not a trait
+                    //~| NOTE: not a trait
                     //~| NOTE: aliases cannot be used for traits
 
 use ImportError; //~ ERROR unresolved