Throw exception when the header cannot be read.

    @Override

    public DictionaryHeader readHeader() throws IOException, UnsupportedFormatException {

        final DictionaryHeader header = mBinaryDictionary.getHeader();

        if (header == null) {

            throw new IOException("Cannot read the dictionary header.");

        }

        if (header.mFormatOptions.mVersion != FormatSpec.VERSION2) {

            throw new UnsupportedFormatException("File header has a wrong version : "

                    + header.mFormatOptions.mVersion);