spellfile.vim: improve error message for missing spellfile

Problem:  If spellfile is missing, then "set spell" in modeline/sandbox
          fails with a non-obvious error.
Solution: Check for sandbox early and give a meaningful error.
          Fixes #11359

Test case:
    # test.latex has `% vim: set spelllang=hu:`
    # no spell file for `hu` yet!
    nvim -u NORC --cmd 'autocmd FileType tex setlocal spell' --cmd 'set modeline' test.latex

Before:
    No spell file for "hu" in utf-8
    Download it?
    Downloading hu.utf-8.spl...
    Error detected while processing /usr/local/share/nvim/runtime/autoload/netrw.vim:
    line  583:
    E12: Command not allowed from exrc/vimrc in current dir or tag search: au WinEnter *^Iif &ft == "netrw"|call s:NetrwInsureWinVars()|endif
    Error detected while processing function spellfile#LoadFile[60]..spellfile#Nread[13]..netrw#NetRead[4]..<SNR>67_NetrwOptionsSave:
    line   66:
    E171: Missing :endif
    Error detected while processing function spellfile#LoadFile[60]..spellfile#Nread:
    line   13:
    E171: Missing :endif
    Error detected while processing function spellfile#LoadFile:
    line   60:
    E171: Missing :endif
    Error detected while processing modelines:
    line    1:
    E12: Command not allowed from exrc/vimrc in current dir or tag search

After:
    Error detected while processing function spellfile#LoadFile:
    line    5:
    E605: Exception not caught: Cannot download spellfile in sandbox/modeline. Try ":set spell" from the cmdline.
    Error detected while processing modelines:
    line    1:
    E12: Command not allowed from exrc/vimrc in current dir or tag search

0 files changed, 0 insertions, 0 deletions