wgpu reports a device out of memory by panicking, and a twelve-frame
merge on a GPU another process is using is where that happens. The panic
unwound the worker, the sender went with it, and the page sat on "Stop"
with every control disabled and nothing to say why — the crash record on
disk was the only sign. Catch the panic and send it as a failure, and
treat a closed channel with no final event as a dead worker too.