This restores go.errors from before 3af0d791 along with a fixed up
version of the bisonerrors AWK script, translated to Go.

However, this means Yyerror needs access to the yacc parser's state,
which is currently private.  To workaround that, add a "state"
accessor method like the Lookahead method added in c7fa3c62.

Update issue #9968.

Change-Id: Ib868789e92fdb7d135442120a392457923e50121
Reviewed-on: https://go-review.googlesource.com/7270Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>