client: return ErrNoEndpoint when none available

In certain cases (for example, if a cluster peer is accessible but it
has no members listed), the httpClusterClient could have an empty set of
endpoints as a result of the Sync. This means that its Do function could
potentially return a nil response and nil error, with catastrophic
consequences for callers.

To be safe (particularly about this latter behaviour), this change
errors in both Sync and Do if no endpoints are available.
Merge pull request #1601 from jonboulle/client

client: return ErrNoEndpoint when none available