Prior to this commit, if a window panic happened, the window would be
left on the screen frozen. Now, the panic is caught and the window is
closed before resuming the panic.
The app event loop has a separate message it receives when a window
panics, and if it's the last window, the process exits with a non-zero
exit code.
This places the burden of handling a panicking window into the
developer's hands: ultimately if the window has a way to communicate
with it, the behavior is being dropped as part of the panic handling,
which ensures any channels the window had will be dropped too.
