titledocument.cpp: check return value from open()

Fixes Coverity CID 709282: Unchecked return value (CHECKED_RETURN)
Calling function "QFile::open(QFlags<QIODevice::OpenModeFlag>)" without checking return value (as is done elsewhere 45 out of 47 times).
No check of the return value of "xmlf.open(QIODevice::OpenMode(_ZN9QIODevice9WriteOnlyE))".
300    xmlf.open(QIODevice::WriteOnly);
301    xmlf.write(doc.toString().toUtf8());