WindowServer: Make SetWindowTaskbarRect tolerant to non-existing windows

There is a window between windows disappearing (e.g. closing or crashes)
and the Taskbar process being notified. So it is entirely possible that
it may call SetWindowTaskbarRect() for a window and/or client id that no
longer exists. As the Taskbar process does not own these windows, this
should not be treated as a misbehaving application request. Instead, just
silently ignore the request. The Taskbar will be notified shortly after
that the window no longer exist and remove it from its list.

Fixes #3494

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