Allow definition of custom 404 error page when device is not reachable

[dominicletz]

The gateway mode should look for 404 BNS property and display that page in case no online device has been found.

Since v256 BNS supports domain propertiers that could be used to place a 404 meta field: https://github.com/diodechain/diode_contract/blob/master/src/BNS.sol

[sc0Vu]

Cool! How about add title, image uri or title to smart contract metadata? (Custom 404 uri redirection might not be safe.)