Problem:
nvim_get_option_value with "filetype" set silently returns incorrect
defaults if autocommands are blocked, like when they're already running.
Solution:
Allow its FileType autocommands to nest: `do_filetype_autocmd(force=true)`.
Also error if executing them fails, rather than silently return wrong defaults.
Endless nesting from misbehaving scripts should be prevented by the recursion
limit in apply_autocmds_group, which is 10.