Wrap field errors in a error span

[edds]

So that they can be styled differently to the label of the error box.

[elliotcm]

I'm down with the span being there but not if it means html_safeing a whole blob of interpolated variables.

I'll refactor it into some nested content_tags on Wednesday if you don't get there first.

[elliotcm]

OK keeping this stuff sensibly html_safe'd is going to be really ugly, so I'll add a warning not to send any user input into the labels, notes, and other options.