Improve QCanBus::instance()->createDevice()

• I'd like to get more information when the above call fails, like "cannot load shared library pcanbasic.dll". For now, the function can only return nullptr.
• From programmers side, it's hard to remember, that backendName is a QByteArray and deviceInterfaceName is a QString. This could be changed to all be QString.

So I'd suggest to have an overload in 5.8 to get the error string:

 QCanBus::instance()->createDevice(
   const QString &backendName,
   const QString &deviceInterfaceName,
   QString *errorString = nullptr);