Text improvement for the error page

[gehinz]

The team BuS suggest changing the text for a unrecognized MAC address to:
"The MAC address currently connected in your room is {}. You need to register this MAC address on our website to unlock the device."
Reason: The old text was not understood by quite a few users, as cheap Routers tend to have a different WAN MAC to the one printed on the router. It was unclear that the MAC shown must be entered on our website.