Add a textual error description for QCanBusFrame::isValid

QCanBusFrame::isValid() already provides a means to check if a frame is valid, i.e. if it can be transmitted over CAN bus.

However, when this check fails, it is hard to know why the frame is invalid. Typical errors may be:

• FrameId exceeds the allowed values
• Payload it too long
• For CAN FD, only discrete values of payload size are allowed

So lets introduce a function to display these errors resp. extend the toString() function to handle these cases too.