add -Werror switch to lupdate to make C++ and QML parsing errors change the exit code

When running lupdate on a qml-file it will not set the return code to 1 in case of QML-parsing errors. This can happen if an invalid QML-file is tested.

lupdate only set the return code to 1 in case of errors when using the .pro-file. But even then it will not react to QML-erros. With QML-error I mean errors due to QQMLEngine-parsing.

I've attached a patch that will solve this problem.