Fix onError for Dev UI streaming

[phillip-kruger]

This PR adds support for errors in streaming methods in Dev UI. It also adds an config option to allow extension developers to switch on the Json RPC log. This is useful for non-core extension developers.

When using streaming in your extension, you can now get errors like this:

this._errorObserver = this.jsonRpc.errorStream().onNext(jsonRpcResponse => {
    console.log(jsonRpcResponse);
}).onError(error => {
    console.log(error);
});

[iocanel]

LGTM and I verify it does solve the reported problem