CANPacker: some exceptions are silent

[sshane]

For example, since we don't propagate exceptions from this function, if you pass certain inputs to make_can_msg (like a list instead of dict), you'll get an exception that Python never knows about. But do we never want to throw an exception? Related: https://github.com/commaai/opendbc/issues/500

https://github.com/commaai/opendbc/blob/3ef35ed2298a3a9d199f9145409547710065884c/can/packer_pyx.pyx#L29-L39

[sshane]

@adeebshihadeh I think this is worth raising an exception for, rather than adding to the valid flag for wrong signals we want to add. What do you think?