Make OpenDirectory fail if the directory doesn't exist

This is in line with what the hardware itself does.

It does this by splitting the initial directory opening into Directory.Open(), which will return false if a stat fails.
Then, Archive::OpenDirectory will return nullptr, and archive.cpp will return an error code .

src/core/file_sys/directory.h

@@ -41,6 +41,12 @@ public:
     Directory() { }
     virtual ~Directory() { }
 
+    /**
+    * Open the directory
+    * @return true if the directory opened correctly
+    */
+    virtual bool Open() = 0;
+
     /**
      * List files contained in the directory
      * @param count Number of entries to return at once in entries