On fmt string with unescaped { note how to escape

On cases of malformed format strings where a `{` hasn't been properly
escaped, like `println!("{");`, present a note explaining how to escape
the `{` char.

src/test/ui/fmt/format-string-error.rs

@@ -0,0 +1,16 @@
+// Copyright 2016 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+fn main() {
+    println!("{");
+    println!("{{}}");
+    println!("}");
+}
+