BpNode: add comment to better reflect what's...

...going on. We are unknown when state calculation fails, not for empty
processes - related git commit message was misleading

library/Businessprocess/BpNode.php

@@ -277,6 +277,7 @@ class BpNode extends Node
                     $e->getMessage()
                 );
 
+                // Failing nodes are unknown
                 $this->state = 3;
             }
         }