wp-688: Implementation of Discovery API: FindCallBack and PendingOperation

It implements methods:
void onError(DOMError error) of interface FindCallBack
void cancel() of interface PendingOperation

Jira issue: WP-688
Jira issue: WP-684
Jira issue: WP-301

Please don't merge the PR until the PR in webinos-jsonrpc2 is merged
https://github.com/webinos/webinos-jsonrpc2/pull/2