OOUIHTMLForm: Prevent duplicate FieldsetLayout wrapping

OOUIHTMLForm: Prevent duplicate FieldsetLayout wrapping

The code in formatSection() assumed it was only called for the
toplevel section (the whole form), while it's actually called
for every subsection too. I think it was written before we added
support for subsections in OOUIHTMLForm.

Move code for toplevel section wrapping to wrapForm().

As a bonus, this also fixes display of custom headers and error
or warning messages for forms with subsections.

Bug: T180535
Change-Id: I6a88184d302a951be78387490404137acde3fa1a