fix(api): not using TRY_WRAP, generic error messages #31595

Problem:
- API functions using `try_start` directly instead of `TRY_WRAP`, do not
  surface the underlying error message, and instead show generic things
  like "Failed to set buffer".
- Error handling code is duplicated in the API impl, instead of
  delegating to the vim buffer/window handling logic.

Solution:
- Use `TRY_WRAP`.

test/functional/api/window_spec.lua

@@ -1664,7 +1664,7 @@ describe('API/win', function()
         autocmd BufWinEnter * ++once let fired = v:true
       ]])
       eq(
-        'Failed to set buffer 2',
+        'Vim:E37: No write since last change (add ! to override)',
         pcall_err(api.nvim_open_win, api.nvim_create_buf(true, true), false, { split = 'left' })
       )
       eq(false, eval('fired'))