Fix filter errors

Prevent error messages from being output to stderr.
Return illegal token when a quoted string is invalid and
capture the error.

Signed-off-by: Derek McGowan <derek@mcgstyle.net>

services/content/contentserver/contentserver.go

@@ -115,7 +115,7 @@ func (s *service) List(req *api.ListContentRequest, session api.Content_ListServ
 
 		return nil
 	}, req.Filters...); err != nil {
-		return err
+		return errdefs.ToGRPC(err)
 	}
 
 	if len(buffer) > 0 {