Improve error handling when Bitmask.Init fails, fixes #855

The segfault happens in gatewaychecker.go.:

// The gateway selector gets populated asynchronously, so this spawns a goroutine that
// checks whether they've been fetched to update status.
func (c connectionCtx) delayCheckForGateways() {
    go func() {
        cnt := 0
        for {
            if cnt > 60*2 {
                break
            }   
            cnt += 1
            time.Sleep(time.Second * 5)
            transport := c.bm.GetTransport()
            locs := c.bm.GetLocationQualityMap(transport)
            if len(locs) != 0 { 
                c.Locations = locs
                updateStatusForGateways()
                break
            }   
        }   
    }() 
}

Having a hardcoded sleep of 5 seconds is bad. The problem is that c.bm is nil. We can use c.IsReady to fix this