Also make setpos("'A", [999, 1, 1, 0]) fail, i.e. return -1 (assuming there is no buffer 999).
Fixes#5713
Background:
`:help setpos()` mentions an argument `"bufnum"` that determines the buffer a mark should be put in.
This argument is respected for uppercase marks, but not for lowercase marks.
This is reasonable (though I personally would like `setpos()` to be able to set marks in other buffers), but the help doesn't mention this anywhere.
It's also strange that attempting to change buffers with `setpos('.', [bufnr('#'), 1, 1, 0])` alerts the user that having a different buffer is an error, while attempting to set a mark with `setpos("'d", [bufnr('#'), 1, 1, 0])` doesn't tell the user that the `"bufnum"` argument is an error.
