Error style in repeats and groups shouldn't have rounded corners

Minor stylistic improvement...

When a field constraint is violated the field background is coloured red. When not in a repeat or group this is shown with subtly rounded corners which looks good. However when it _is_ in a repeat or group the question runs right up to the edge of the border of the group and the rounded corners don't look good.

Remove the rounded corners on error blocks when a border exists, so it looks like this:

NB: on the smallest screens the repeats and groups don't have a border so the error should be rounded again...

Start by locating the styles for error blocks and the repeat or group borders. Check the responsive behavior at the smallest screen size, then verify that error blocks lose rounded corners only when a border is present and retain them when the border is absent.

Written by the indexing model from the issue text.