In this patch we lower the log level of the failures for the three calls
to unlink() in networkstatus_set_current_consensus(). These errors might
trigger on Windows because the memory mapped consensus file keeps the
file in open state even after we have close()'d it. Windows will then
error on the unlink() call with a "Permission denied" error.

The consequences of ignoring these errors is that we leave an unused
file around on the file-system, which is an easier way to fix this
problem right now than refactoring networkstatus_set_current_consensus().

See: https://bugs.torproject.org/29930