Allow Praefect's ServerInfo RPC to succeed even if the internal gitaly node calls fail

Since the ServerInfo is for informational purposes, it doesn't help to fail the whole RPC if one of the internal gitaly calls results in an error. We should return whatever info we can.

Currently we use the errgroup package, which will fail fast and return an error as soon as one of the goroutines experiences an error. https://godoc.org/golang.org/x/sync/errgroup#Group.Go