Fix in case of internal server error

When battle.net has an “”Internal server error” it will return an error
message as JSON. If this happens when Colligate looks up item  names it
will no longer store the error instead of actual item names and JSON.
Now we simply skip the item name and wait for next update.
Item lookup crash fix

If the Downloader class caught an exeption while looking up an item’s
name it would not return anything. Now we check  whether we get a
return value from getItemJSON method.
alternate item lookup

If battle.net returns “”unable to get item information.” we will try
and look up the item’s name on Wowhead instead. This does not provide
any JSON however we can get the name of an item that is no longer
obtainable in the game.