tostring() the result in warning vim.notify calls

[ndom91]

Ran into this issue recently when opening the file picker:

  Error  E5108: Lua: ...al/share/nvim/lazy/fff.nvim/lua/fff/file_picker/init.lua:159: attempt to concatenate local
'result' (a userdata value)
stack traceback:
    ...al/share/nvim/lazy/fff.nvim/lua/fff/file_picker/init.lua:159: in function 'get_scan_progress'
    ...91/.local/share/nvim/lazy/fff.nvim/lua/fff/picker_ui.lua:2686: in function 'monitor_scan_progress'
    ...91/.local/share/nvim/lazy/fff.nvim/lua/fff/picker_ui.lua:2587: in function 'open'
    .../ndom91/.local/share/nvim/lazy/fff.nvim/lua/fff/main.lua:41: in function 'live_grep'
    /Users/ndom91/.config/nvim/lua/plugins/fff.lua:53: in function </Users/ndom91/.config/nvim/lua/plugins/fff.lua:52>

And noticed that many of the vim.notify calls didn't tostring() the result. Updated here to help avoid similar errors as above