Bug1829, more: check all errors flushing and closing export files...

... MP3 export already did this, the other five needed work.

src/FileIO.h

@@ -32,7 +32,7 @@ class FileIO
 
    bool IsOpened();
 
-   void Close();
+   bool Close();
 
    wxInputStream & Read(void *buffer, size_t size);
    wxOutputStream & Write(const void *buffer, size_t size);
@@ -41,7 +41,7 @@ class FileIO
    wxString mName;
    FileIOMode mMode;
    std::unique_ptr<wxInputStream> mInputStream;
-   std::unique_ptr<wxOutputStream> mOutputStream;
+   std::unique_ptr<wxFFileOutputStream> mOutputStream;
    bool mOpen;
 };