encoding/csv: update ErrQuote message

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/72794
Reviewed-by: Avelino <t@avelino.xxx>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/encoding/csv/reader.go

@@ -84,7 +84,7 @@ func (e *ParseError) Error() string {
 var (
 	ErrTrailingComma = errors.New("extra delimiter at end of line") // Deprecated: No longer used.
 	ErrBareQuote     = errors.New("bare \" in non-quoted-field")
-	ErrQuote         = errors.New("extraneous or missing \" in field")
+	ErrQuote         = errors.New("extraneous or missing \" in quoted-field")
 	ErrFieldCount    = errors.New("wrong number of fields")
 )