Handle selection transfer errors earlier

Handle selection transfer errors earlier

* src/xselect.c (x_decline_selection_request, struct transfer)
(struct x_selection_request, x_cancel_selection_transfer)
(x_start_selection_transfer, x_continue_selection_transfer):
Give the right serial to x_ignore_errors_for_next_request.
(x_handle_selection_error): New function.
(x_reply_selection_request): Give the right serial to
x_ignore_errors_for_next_request.
* src/xterm.c (x_ignore_errors_for_next_request): New arg
`selection_serial'.  All callers changed.
(x_error_handler): Call selection error handler.
* src/xterm.h (struct x_failable_request): New field
`selection_serial'.