gui: Ensure tx send error highlight is visible

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.

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;
             }
         }