Improve error reporting

Before

The error handling just reports "you need to review FieldName". Also, FieldName is not html escaped so there's a mild possibility of XSS.

After

As well as listing the fields, it shows the friendly/short (and usually useless!) error message and the verbose error which is useful in diagnosing the problem.

Screenshot below (with Aah theme - it's only the text of the message that this MR changes)