qmllint: provide better error message when passing a directory instead of a file to -i option

I accidentally passed a directory when I should have passed a file:

-i /home/mitch/dev/qt5-dev2-debug/qtbase/qml

The error was:

QIODevice::read (QFile, "/home/mitch/dev/qt5-dev2-debug/qtbase/qml"): device not open
1:1: Expected token `numeric literal'

It would be more user friendly to warn the user that -i expects a file and not a directory.