Catch an error that is reported, but silently ignored during CI testing

[freakboy3742]

Close inspection of a CI run for the Winforms testbed shows the following:

tests\widgets\test_table.py::test_headerless_column_changes PASSED       [ 75%]
tests\widgets\test_table.py::test_remove_all_columns Traceback (most recent call last):
  File "D:\a\toga\toga\testbed\build\testbed\windows\app\src\app_packages\toga_winforms\widgets\table.py", line 176, in _new_item
    icon = icon(self._accessors[0])
  File "D:\a\toga\toga\testbed\build\testbed\windows\app\src\app_packages\toga_winforms\widgets\table.py", line 90, in winforms_retrieve_virtual_item
    e.Item = self._new_item(e.ItemIndex)
  File "D:\a\toga\toga\testbed\build\testbed\windows\app\src\app_packages\toga_winforms\libs\wrapper.py", line 22, in __call__
    return function(*args, **kwargs)
   at Python.Runtime.PythonException.ThrowLastAsClrException() in /tmp/build-via-sdist-h3j67_wc/pythonnet-3.0.3/src/runtime/PythonException.cs:line 53

   at Python.Runtime.Dispatcher.TrueDispatch(Object[] args) in /tmp/build-via-sdist-h3j67_wc/pythonnet-3.0.3/src/runtime/DelegateManager.cs:line 341

   at Python.Runtime.Dispatcher.Dispatch(Object[] args) in /tmp/build-via-sdist-h3j67_wc/pythonnet-3.0.3/src/runtime/DelegateManager.cs:line 208

   at __System_Windows_Forms_RetrieveVirtualItemEventHandlerDispatcher.Invoke(Object , RetrieveVirtualItemEventArgs )

   at System.Windows.Forms.ListView.OnRetrieveVirtualItem(RetrieveVirtualItemEventArgs e)

   at System.Windows.Forms.ListView.WmReflectNotify(Message& m)

   at System.Windows.Forms.ListView.WndProc(Message& m)

   at System.Windows.Forms.NativeWindow.Callback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)
list index out of range
PASSED              [ 75%]
tests\widgets\test_table.py::test_cell_icon PASSED                       [ 76%]

tests\widgets\test_table.py::test_remove_all_columns is passing but raising an exception that is being ignored. The error is caused when the all columns are removed from the table, it no longer has accessors, so looking up column 0 to render the table's icon fails. However, because this exception occurs in a callback, it doesn't surface as a full error.

This PR catches and ignores the error.

PR Checklist:

• All new features have been tested
• All new features have been documented
• I have read the CONTRIBUTING.md file
• I will abide by the code of conduct

[mhsmith]

This could hide other IndexErrors from inside icon(), so in this case I think it's better to ask for permission rather than forgiveness.