[electrum] minor cleanup in Transaction.verify_signature

Summary:
Remove an unnecessary level of indentation.
Merge two except blocks that do the same thing, which allows removing ecdsa includes in this module (we don't need to know about such implementation details in this scope).

Depends on D16301

Test Plan: python test_runner.py

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16302