CRM_Core_Form::validate returns incorrectly

Using hook_civicrm_validate, a developer could unset validation errors, but the form will still not pass to submit to submission because CRM_Core_Form::validate() returns an out-dated value.

Instead, the function return in the same way as it's parent, by checking the count of _errors. See HTML_Quickform::validate().

The attached one-line patch fixes this, and makes hook_civicrm_validate immensely more useful.