fix(api): generic error messages, not using TRY_WRAP #31596

Problem:
- API functions using `try_start` directly, do not surface the
  underlying error message, and instead show generic messages.
- Error-handling code is duplicated in the API impl.
- Failure modes are not tested.

Solution:
- Use `TRY_WRAP`.
- Add tests.

test/functional/lua/vim_spec.lua

@@ -3955,6 +3955,17 @@ stack traceback:
       eq(win2, val)
     end)
 
+    it('failure modes', function()
+      matches(
+        'nvim_exec2%(%): Vim:E492: Not an editor command: fooooo',
+        pcall_err(exec_lua, [[vim.api.nvim_win_call(0, function() vim.cmd 'fooooo' end)]])
+      )
+      eq(
+        'Error executing lua: [string "<nvim>"]:0: fooooo',
+        pcall_err(exec_lua, [[vim.api.nvim_win_call(0, function() error('fooooo') end)]])
+      )
+    end)
+
     it('does not cause ml_get errors with invalid visual selection', function()
       -- Add lines to the current buffer and make another window looking into an empty buffer.
       exec_lua [[