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
What's the reason for ParallelEffects vs a list of things? I'm guessing "you don't actually want to run all of this in parallel", but I figured that's what parallel's concurrency limiting is for?