more progress on bug 113

better handling of error conditions in all BuildFromXML methods, per comment: 

// BuildFromXML methods should always return a BlockFile, not NULL,  
// even if the result is flawed (e.g., refers to nonexistent file), 
// as testing will be done in DirManager::ProjectFSCK().

src/ondemand/ODDecodeTask.h

@@ -40,9 +40,6 @@ class ODFileDecoder;
 class ODDecodeTask:public ODTask
 {
  public:
-   // Constructor / Destructor
-
-   /// Constructs an ODTask
    ODDecodeTask();
    virtual ~ODDecodeTask(){};