fix(autoread): handle autocmd errors

Problem:
Any random ftplugin or other autocmd, can throw an error when
`:checktime` reloads a buffer. This causes a trace which makes it look
like an issue with `autoread.lua`.

    vim.schedule callback: …/runtime/lua/nvim/autoread.lua:146:
    FileType Autocommands for "*"..function <SNR>1_LoadFTPlugin[20] ..script
    …/runtime/ftplugin/help.lua: Vim(runtime):E5113: Lua chunk:
    …/runtime/lua/vim/treesitter.lua:216: Index out of bounds
    stack traceback:
    [C]: in function 'nvim_buf_get_text'
    …/runtime/lua/vim/treesitter.lua:216: in function 'get_node_text'
    …/runtime/lua/vim/treesitter/query.lua:558: in function 'handler'
    …/runtime/lua/vim/treesitter/query.lua:843: in function '_match_predicates'
    …/runtime/lua/vim/treesitter/query.lua:1082: in function '(for generator)'
    …/runtime/ftplugin/help.lua:91: in function 'runnables'
    …/runtime/ftplugin/help.lua:124: in main chunk
    [C]: in function 'checktime'
    …/runtime/lua/nvim/autoread.lua:146: in function <…/runtime/lua/nvim/autoread.lua:138>

Solution:
Use pcall() and surface the error via nvim_echo.

runtime/lua/nvim/autoread.lua

@@ -140,13 +140,19 @@ local function ensure_watcher(bufnr)
           set_pending(bufnr, false)
           return
         end
-        vim.cmd.checktime(bufnr)
+        -- Use pcall: autocmds (e.g. ftplugin) may throw during reload for any reason.
+        local ok, err = pcall(vim.cmd.checktime, bufnr) ---@type any, any
         set_pending(bufnr, false)
         -- On rename events (e.g. atomic save by another editor), the watcher
         -- is now stale (watching the old inode). Re-establish it.
         if change_type ~= watch.FileChangeType.Changed then
           ensure_watcher(bufnr)
         end
+        if not ok then
+          vim.api.nvim_echo({
+            { ('autoread: :checktime failed for buffer %d: %s'):format(bufnr, err) },
+          }, true, { err = true })
+        end
       end)
     end)
   end)