Ignore/log errors while fetching servers for groups

This means that if there's an error such as a tenant being deleted (which happened in production), we'll still be able to collect metrics.

This is based on the pure-server-info-2 branch, so to look at the diff check out https://github.com/rackerlabs/otter/compare/pure-server-info-2...ignore-metrics-errors
It's really just glue for logging -- if we had a logging function that took an exc_info tuple instead of a Failure then we wouldn't have to mention Twisted stuff here. It could probably be done a bit better, but I think it's okay for convenience.