The ErrQuote variable is only returned when a parsing error
occurs within a quoted string. Make that clear in the message.

Change-Id: I06ad5a9edb41afedde193c4f8b93551bb8342bbb
Reviewed-on: https://go-review.googlesource.com/72794Reviewed-by: Avelino <t@avelino.xxx>
Reviewed-by: Ian Lance Taylor <iant@golang.org>