some godoc cleanup:

- simplified dealing with parse errors: no need to intersperse them in the source
- improve visibility of highlighted identifiers by showing them in bold

R=rsc
https://golang.org/cl/163051

lib/godoc/source.html

@@ -0,0 +1,13 @@
+<!--
+	Copyright 2009 The Go Authors. All rights reserved.
+	Use of this source code is governed by a BSD-style
+	license that can be found in the LICENSE file.
+-->
+
+{.section Error}
+	<p>
+	<span class="alert" style="font-size:120%">{@|html}</span>
+	</p>
+{.or}
+	<pre>{Source|html}</pre>
+{.end}