Show new style Windows directory selector even for non-existent paths.

Passing an invalid initial directory to wxDirDialog resulted in using the old
style Windows directory selector dialog instead of the new style (file dialog
like) one, which was unintentional, as incorrect initial directory shouldn't
prevent the dialog from being shown.

Fix this by handling ERROR_FILE_NOT_FOUND error specifically and still showing
the new dialog, just without the (non existent) initial directory value, in
this case.

Closes #16430.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@77081 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775