Don't give error message for non-existent paths in wxFileSystemWatcher.

Don't give error message for non-existent paths in wxFileSystemWatcher.

The file we're trying to watch might have been just deleted, this is an
unavoidable (and not only in theory but also in practice when watching busy
directories) race condition so don't give an error message if this does happen
but simply return false from Add().

Closes #15051.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@73591 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775