Problem:
`nvim --listen` does not error on EADDRINUSE. #30123
Solution:
Now that `$NVIM_LISTEN_ADDRESS` is deprecated and input *only* (instead
of the old, ambiguous situation where it was both an input *and* an
output), we can be fail fast instead of trying to "recover". This
reverts the "recovery" behavior of
704ba4151e, but that was basically
a workaround for the fragility of `$NVIM_LISTEN_ADDRESS`.
96128a5076