rpc client: Handle case if aepp opened without wallet

[davidyuk]

window.parent is equal to window if a web page opened as usual. If we accidentally start dapp in this way (also not setting parent option), then rpc/client will send a hello to itself and fail with exception:

Uncaught TypeError: Cannot destructure property resolve of 'undefined' or 'null'. at receive (client.js?aafa:56)

I think much cleaner for dapp developers to handle this case separately. In that case this PR makes dapp to throw:

Uncaught (in promise) Error: rpc client: Can't send messages to itself at Object._callee$ (client.js?6b57:46)