"Can only throw objects in provision.config.inc, line 75"

While developing a new service for Provision, I ran across the following error message:

Drush command terminated abnormally due to an unrecoverable error.                                                                                                 [error]
Error: Can only throw objects in /opt/aegir/.drush/provision/provision.config.inc, line 75

The offending code is:

function __construct($context, $data = array()) {
    if (is_null($this->template)) {
      throw(exception);  // Line 75
    }
    ...

I'm guessing this is supposed to be something like:

function __construct($context, $data = array()) {
    if (is_null($this->template)) {
      drush_set_error('PROVISION_INTERNAL_ERROR', dt("No template specified: %class", array('%class' => get_class($this))));
      return;
    }
    ...

Assuming this is the correct way to handle this issue, attached is a patch to corrects this oversight.