Update demo smartwizard2-validation.php to hide stale validation error msgs

Once 'showMessage' displays an error message, the message persists after the error has been cleared, even into the next step. This change will call 'hideMessage' to  hide the error message once the error has been cleared.
Merge pull request #19 from mhutchin/patch-1

Update demo smartwizard2-validation.php to hide stale error msgs