improve Form redirect handling.

Previously, the Form lifecycle was cut short if there was a redirect from the form handler (see FIXME in removed code). This aims to correct this issue while maintaining previous functionality where desired.
correct redirect handling in Form for MailerModule
add exit command.
Merge pull request #1509 from craigh/formRedirectImprovement

Form redirect improvement
correct variable name (_IsValid -> _isValid) to match use elsewhere
Merge pull request #1510 from craigh/correctVarName

correct variable name (_IsValid -> _isValid) to match use elsewhere