Thanks Manuel for the latest update of the class.

I noticed a particular bug fix:

- Fixed bug of adding custom input validation errors twice when
ShowAllErrors option is set.

I picked this up a while ago but didn't go through the process of
tracing where the duplication was coming from. Due to time constraints
I wrote it off as an error I'd made (as the custom input I was
creating used 2 inputs). So I appreciate that it's now fixed.

However it's not totally solved. If ErrorMessagePrefix is set (i.e.
not '') then the prefix is still outputted twice.
i.e. the error message is now no longer outputted twice, however the
ErrorMessagePrefix is outputted twice.

Hope it can be tracked down.

Cheers, Saul