clients: ivi-shell-user-interface, Check return value of wl_display_dispatch.

> So here is the main loop, and it gracefully ignores all error
> conditions, which means that this program never exits if it gets
> disconnected - instead it starts eating 100% CPU until forcefully
> killed.
> Now I see why you had the server kill this client. You'd better fix this
> main loop instead.
> For guidance, see what simple-shm.c does.

Signed-off-by: Nobuhiko Tanibata <NOBUHIKO_TANIBATA@xddp.denso.co.jp>