Rename GetAllGateways to FetchAllGateways and change return values of FetchAllGateways

After removing `Bitmask.ReloadFirewall()` there are no calls to `b.api.GetAllGateways()`
where the calling function actually need the gateways as return values. So I renamed the
function to FetchAllGateways. The function now only returns an error (instead of
`([]bonafide.Gateway, error)`). This simplifies the code of v5 a bit, as
we get rid of yet another function/abstraction. In v3 `gatewayPool.getAll()`
now also returns just an error.