New exception type.

core/exceptions.class.php

@@ -9,3 +9,11 @@ class SCoreException extends Exception {}
  */
 class PermissionDeniedException extends SCoreException {}
 
+/**
+ * Class ImageDoesNotExist
+ *
+ * This exception is used when an Image cannot be found by ID.
+ *
+ * Example: Image::by_id(-1) returns null
+ */
+class ImageDoesNotExist extends SCoreException {}