FileSys: split the constructor into an Open method, in order to notify the opener something went wrong.

Kernel: Return an invalid handle to OpenFile when it failed to open.

src/core/file_sys/file.h

@@ -18,6 +18,12 @@ public:
     File() { }
     virtual ~File() { }
 
+    /**
+     * Open the file
+     * @return true if the file opened correctly
+     */
+    virtual bool Open() = 0;
+
     /**
      * Read data from the file
      * @param offset Offset in bytes to start reading data from