[backport#14690] Throw error if CPubKey is invalid during PSBT keypath serialization

Summary:
Related to #14689

We should catch this error before attempting to deserialize it later.

Depends on D6029

This is a backport of Core PR14690

Test Plan:

cmake .. -GNinja -DENABLE_WERROR=ON
ninja check-all

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6030