Change a qFatal to a qWarning

There are valid uses cases how to use Qt without installing
any fonts (e.g. by using an application font), so let's
make the warning non fatal.

Task-number: QTBUG-29192
Change-Id: I5684331b687bef4b8a54be1f68303c80aa8d4372
Reviewed-by: Paul Olav Tvete <paul.tvete@digia.com>
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@digia.com>

src/platformsupport/fontdatabases/basic/qbasicfontdatabase.cpp

@@ -96,8 +96,9 @@ void QBasicFontDatabase::populateFontDatabase()
     QString fontpath = fontDir();
 
     if(!QFile::exists(fontpath)) {
-        qFatal("QFontDatabase: Cannot find font directory %s - is Qt installed correctly?",
+        qWarning("QFontDatabase: Cannot find font directory %s - is Qt installed correctly?",
                qPrintable(fontpath));
+        return;
     }
 
     QDir dir(fontpath);