[electrum] properly handle point at infinity and point not on curve

Summary:
Raise a custom error for points not on the curve. Support a special *point a inifinity* value for ECPubkey (will be needed in the next diff for Cash Fusion code)

This is a backport of https://github.com/spesmilo/electrum/commit/59c1d03f018026ac301c4e74facfc64da8ae4708

Additional tests from:

• electrumabc_plugins/fusion/tests/test_pedersen.py

Depends on D16676

Test Plan: python test_runner.py

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Subscribers: bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D16681