[electrum] minor cleanup of scan_beyond_gap.py

Summary:

• Split done_signal(Optional[tuple[int, int]], Optional[Exception]) into done_signal(int, int) and error_signal(Exception) so we don't have to inspect the params to check for None and we don't have to unpack a tuple.
• give the end indices directly to add_address so it doesn't have to scan a list of tuples to find these indices.

Test Plan: Try the tool in Electrum ABC with a deterministic wallet (menu Wallet > Scan More Addresses)

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15848