Merge #16694: gui: Ensure transaction send error is always visible

a4765bd gui: Ensure tx send error highlight is visible (bpay)

Pull request description:

  Rebased and squashed #14956.

  > 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. This results in a confusing lack of error feedback.

  > Avoid this problem by ensuring the first field containing an error is scrolled into view when Send is clicked.

  You can see the behavior here: https://imgur.com/a/QZG5TQc

  How to test:
  Add a few recipients and give any of them an invalid address or amount. Scroll the invalid recipient out of view and hit Send. With this change, the GUI will scroll to show the invalid recipient, with master it will not, "hiding" the error.

ACKs for top commit:
  jonatack:
    Tested ACK a4765bd on Linux Debian with Qt 5.11.3. Change is that I had added an inadvertent typo in my make bash alias; fixed.
  hebasto:
    ACK a4765bd, tested on Debian 9.9 with system Qt 5.7.1.

Tree-SHA512: a5653ca44d6d540214bdb424b0b75a06a5872cff41b0cd8cffd9cef99ebf04a17a3652e561139ac75315b39c3347e5f7ae304fa35e14b48bdae4768a416df9b0

Author: fanquake

src/qt/sendcoinsdialog.cpp

@@ -230,8 +230,9 @@ void SendCoinsDialog::on_sendButton_clicked()
             {
                 recipients.append(entry->getValue());
             }
-            else
+            else if (valid)
             {
+                ui->scrollArea->ensureWidgetVisible(entry);
                 valid = false;
             }
         }