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/parseerror.html

@@ -1,10 +0,0 @@
-<!--
-	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.
--->
-
-<pre>
-{.repeated section list}
-{src|html}{.section msg}<b><span class="alert">«{msg|html}»</span></b>{.end}{.end}
-</pre>