A failures logger.
Updates a list of failed links.
Updates a list of failed links. If a link already on the list
is found to be working, it is removed. After n days
we only have links on the list which failed within those n days.
Intialize with old failures data (if found).
Write failures file.
Add invalid url to failures, delete valid url from failures.
Read a previously stored failures from file fd.
Write the failures file.