W3C logger updates

* Open log file in 'a' mode every time, which will create it if it
  doesn't exist, and append to it if it does exist. Previously it was
  overwriting the file if it existed when master was restarted ('w' mode).
* Protects against the case where the path leading up to the log file
  doesn't exist, which would cause the entire app to crash. I'm open to
  finding a better way to handle this than what I have here.