Problem:
`nlua_pcall()` references `_G.debug.traceback`. If user code deletes it
or breaks it some other way, various Lua features are broken.
_G.debug = nil
vim.schedule(function() end)
vim.wait(100)
E5113: Lua chunk: attempt to index a nil value
stack traceback:
[C]: in function 'loop_poll'
[string "vim/_core/editor"]:176: in function 'wait'
crash.lua:3: in main chunk
PANIC: unprotected error in call to Lua API (attempt to index a nil value)
Solution:
Check `_G.debug.traceback` before using it as errfunc. If it's broken,
omit the traceback and say so in the error message.
Note: We could cache `_G.debug` in LUA_REGISTRYINDEX on startup, but
that would prevent plugins from providing custom functionality there
(and we happen to do so in `tui_spec.lua` for example).
02b0c80422