Context:
Nvim catches errors from the user's `on_exit` and rpc handler callbacks
and prints the error message.
Problem:
Printing the error message uses Nvim api functions. But callbacks
mentioned above run in `:h lua-loop-callbacks` where most of `vim.api`
is not allowed, so Nvim itself raises error.
Solution:
`vim.schedule()` the error reporting when necessary.
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