Handle errors while sending client events asynchronously

Handle errors while sending client events asynchronously

* src/xterm.c (xm_send_drop_message)
(xm_send_top_level_enter_message, xm_send_drag_motion_message)
(xm_send_top_level_leave_message, x_dnd_send_enter)
(x_dnd_send_position, x_dnd_send_leave, x_dnd_send_drop): Avoid
sync to check for errors while sending client events.
(x_dnd_begin_drag_and_drop, handle_one_xevent, XTread_socket):
Clean up failable requests.
(x_request_can_fail): New functions.
(x_clean_failable_requests, x_ignore_errors_for_next_request)
(x_uncatch_errors): Clean up failable requests.
(x_error_handler): If a request is allowed to fail, just return.
(x_term_init): Set up new pointer.
* src/xterm.h (N_FAILABLE_REQUESTS): New macro.
(struct x_display_info): New field `failable_requests' and
associated next pointer.