If you enable logging but specify a dir not writable you are screwed:
1) in the ui you get error 500 so you cannot change back the setting
2) you do not see the error at all in the logs (even apache logs) since logging fails

The attached patch makes it a bit more robust by falling back to logging to stderr, which is important (e.g. filesystem mounted readonly or other weird cases). The UI should also try to prevent this situation, but that is a separate bug.