This extends cmd/yacc with support for

	%error { tokens } : message

syntax to specify custom error messages to use instead of the default
generic ones.  This allows merging go.errors into go.y and removing
the yaccerrors.go tool.

Updates #9968.

Change-Id: I781219c568b86472755f877f48401eaeab00ead5
Reviewed-on: https://go-review.googlesource.com/8563Reviewed-by: Russ Cox <rsc@golang.org>