Rc2 tweaks

A quick change to the host to change sector file permissions from 0700 to 0400, tests seem to be coping with it well. Also added a logging statement to the storage manager if a sector write fails, as currently that failure is recorded but no reason is given for the failure.

Finally, I updated the hostdb to de-prioritize hosts that don't have much storage remaining. This is because those hosts have limited utility, especially given that contract fees are so high. I'm not sure if I calibrated the numbers correctly, the first penalty hits at 200GB (it's not too severe), and then below 50GB you get hit really hard. Below 1GB you are essentially ignored as a host.

I'm re-running the hostdb on my local machine to make sure this doesn't affect prices too dramatically. I'm not actually sure what percentage of hosts have more than 200GB available, but my initial checks suggest it's well above 50%, which is where it needs to be for this change to not be disruptive.