Problem: there are new ways to escape textlock or break the cmdwin in
nvim_win_set_config and nvim_tabpage_set_win.
Solution: fix them. Use win_goto to check it in nvim_tabpage_set_win and use the
try_start/end pattern like with similar functions such as nvim_set_current_win
(which uses the existing msg_list, if set).
Careful not to use `wp->handle` when printing the window ID in the error message
for nvim_tabpage_set_win, as win_goto autocommands may have freed the window.
On a related note, I have a feeling some API functions ought to be checking
curbuf_locked...