Rename GetAllGateways to FetchGateways and change return values of FetchGateways

After removing `Bitmask.ReloadFirewall()` there are no calls to `b.api.GetAllGateways()`
where the calling function acutally need the Gateways as return values. So I renamed the
function to FetchGateways. The function now only returns an error (instead of
`([]bonafide.Gateway, error)`). This simplifies the code of v5 a bit.
In v3 `gatewayPool.getAll()` now also returns just an error. `p.getGatewaysByTimezone`
and `p.getGatewaysFromMenshen` are also used by `getBest()` so I didn't touch that.
We want to get rid of v3 anyway...