fixed extra constraint violations

Hi guys,

This change solves a bug which adds extra constraint violations to the execution context. From the moment when there is a constraint violation in the context, each call to *ErrorElement#validate* will add new constraint violation to the context and when the method is called without second argument  *$messageTemplate* the default value *NULL* will result in empty string error message.

This might be related to issue #275 and issue #617.

Cheers,
Stan