Fix error handling for XCB Xlib

Fix error handling for XCB Xlib

* 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_compute_toplevels)
(x_dnd_send_enter, x_dnd_send_position, x_dnd_send_leave)
(x_dnd_send_drop, handle_one_xevent, x_catch_errors_with_handler)
(x_request_can_fail, x_clean_failable_requests)
(x_ignore_errors_for_next_request, x_stop_ignoring_errors)
(x_uncatch_errors, x_check_errors, x_had_errors_p, x_error_handler)
(frame_set_mouse_pixel_position, x_focus_frame): Record serial
sequences instead of simply the next request when ignoring a
single request.  Use XNextRequest instead of NextRequest, since
the latter is unreliable when using Xlib built with XCB.

* src/xterm.h (struct x_failable_request): New struct..
(struct x_display_info): Make failable request variables the
right type.