Use replacement variables for errors recorded in watchdog

Currently we record MC API errors like this -

watchdog('mailchimp', $q->errorMessage(), NULL, WATCHDOG_ERROR); 

IMO we should do this instead -

watchdog('mailchimp', 'MCAPI Error: %errmsg', array('!errmsg' => $q->errorMessage()), WATCHDOG_ERROR); 

* This protects against potential injection attacks via MCAPI (results from MC API are external input after all).
* The MCAPI prefix (along with the existing $type value) also makes the original source of the error message clearer.