Restore form validation error message removed in deformdemo test fixes

A commit fixing deformdemo tests seems to have accidentally removed some form validation error explanation text:

https://github.com/Pylons/deform/commit/772b8ce8664cb03f1fb2c893175c47a75a496f7c#diff-67b2f092186b770c5b490505b61462c5

I tried to get the deformdemo tests running to check my work, but my test browser freezes during the run.