throw an error if the specified `--server` is not in server list

Fixes https://github.com/librespeed/speedtest-cli/issues/55

Testing

cat <<EOF > /tmp/server-list.json
[
  {
    "pingURL": "empty.php", 
    "name": "Amsterdam, Netherlands (Clouvider)", 
    "sponsorURL": "https://www.clouvider.co.uk/", 
    "getIpURL": "getIP.php", 
    "ulURL": "empty.php", 
    "sponsorName": "Clouvider", 
    "id": 51, 
    "dlURL": "garbage.php", 
    "server": "//ams.speedtest.clouvider.net/backend"
  }
]
EOF

librespeed / speedtest-cli

throw an error if the specified `--server` is not in server list #56