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_romfs.cpp

@@ -17,6 +17,14 @@ File_RomFS::File_RomFS() {
 File_RomFS::~File_RomFS() {
 }
 
+/**
+ * Open the file
+ * @return true if the file opened correctly
+ */
+bool File_RomFS::Open() {
+    return false;
+}
+
 /**
  * Read data from the file
  * @param offset Offset in bytes to start reading data from