Could not write error to log file

While debugging for this issue https://github.com/Codeception/Codeception/issues/4871, I keep getting message `Could not write error to log file`.
When I traced the cause, it's from here.

https://github.com/Codeception/c3/blob/c7348bbc82da82834fe237c5fb754003ac0fe782/c3.php#L36-L43

Apparently the `c3tmp` directory is always emptied by `__c3_clear()` when the test is executed.

https://github.com/Codeception/c3/blob/c7348bbc82da82834fe237c5fb754003ac0fe782/c3.php#L288-L291

This cause problem because `is_writable` returns true if the file exists and is writable.

http://php.net/manual/en/function.is-writable.php