Send 504 Gateway Timeout for vendor unreachable
Send 504 Gateway Timeout for vendor unreachable

As discussed in #445

malditogeek/shields,pine613/shields,freezy-sk/shields,malditogeek/shields,pine613/shields,badges/shields,badges/shields,mahirk/shields,freezy-sk/shields,badges/shields,tomap/shields,mahirk/shields,tomap/shields