pkg: fix SetFlagsFromEnv behaviour

This function was fundamentally buggy, as a panic could be trivially triggered
by setting the wrong environment variable (e.g. ETCD_BIND_ADDR=foo). Instead,
let's propagate the error and present it to the user in a cleaner way.


noted via #1640
we don't give name to returned values generally.
Or it makes godoc look ugly.
https://code.google.com/p/go-wiki/wiki/CodeReviewComments#Named_Result_Parameters