gui: Ensure tx send error highlight is visible

Summary:
If sending to multiple recipients and one of the recipient fields is malformed,
the highlighted field may not be visible due to being scrolled out of view,
leading to a confusing lack of error feedback when clicking Send. To avoid this
problem ensure the first field containing an error is scrolled into view
when Send is clicked.

Backport of Core PR16694

Test Plan:
ninja && ninja check
In bitcoin-qt, create a transaction with multiple recipients, add a mistake to
an address that is scrolled out of view, click send.

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7904