Clear any input_errors for an input after it is signed

Summary:
Make sure that there are no errors set for an input after it is signed.
This is useful for when there are multiple ScriptPubKeyMans. Some may
fail to sign, but one may be able to sign, and after it does, we don't
want there to be any more errors there.

Backport of Core PR18115 part [9/9] : https://github.com/bitcoin/bitcoin/pull/18115/commits/d2774c09cfcc6c5c967d40bb094eabc8c0bdb6bf

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8103