Use best holdout loss when reporting final 'average loss' when holdout enabled

Even with multiple passes the 'average loss' in final output was still
progressive validation which does not make sense.

This pull-request shows best holdout loss if holdout is enabled with a
customary 'h' appended.

I have also encountered the too small diff width issue in testcases again and
while debugging I have switched the printed diff to the same side-by-side diff
format that is used internally for lenient float comaprison. This made
differences more obvious and debugging easier.