[chronik] Improve error handling of the electrum interface

Summary:
The current solution will wait forever when a future is failing which is not the expected behavior. Use start_block() instead to properly catch the errors.

Depends on D19810.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Subscribers: PiRK

Differential Revision: https://reviews.bitcoinabc.org/D19801