Type `XXX` not configured for class `CM_Mail`

This error shows up in the error log, from email-view-tracking requests. Seems like email clients (or other clients) are sending invalid types.
I suggest we make this a warning.
@fauvel @mariansollmann agree?

```
CM_Class_Exception_TypeNotConfiguredException: Type `104` not configured for class `CM_Mail`. in /home/application/releases/20140918120142/vendor/cargomedia/cm/library/CM/Class/Abstract.php on line 38
    0. {main} /home/application/serve/public/index.php:0
    1. CM_Response_Abstract->process() /home/application/releases/20140918120142/public/index.php:10
    2. CM_Response_EmailTracking->_process() /home/application/releases/20140918120142/vendor/cargomedia/cm/library/CM/Response/Abstract.php:42
    3. CM_Action_Email->__construct('VIEW', SK_User(1186029), 104) /home/application/releases/20140918120142/vendor/cargomedia/cm/library/CM/Response/EmailTracking.php:11
    4. CM_Class_Abstract->_getClassName(104) /home/application/releases/20140918120142/vendor/cargomedia/cm/library/CM/Action/Email.php:16
```