Improper error handling for parsing of certificate PEM in fleetctl

In https://github.com/kolide/fleet/blob/master/server/service/client.go#L53 we use `errors.Wrap` inside a statement that returns only a `bool` error status. `errors.Wrap` returns `nil` if the provided error is `nil`. If execution has made it to this point, the `err` variable is certain to be `nil`.

The function returns `nil, nil`, leading to a segfault later when the client is used.