Consolidate the error messages handling in the CAN plugins

Right now we have two means to report errors during operation:

1. print log messages with qCDebug/qCWarning
2. call QCanBusDevice::setError(), which emits a signal

For now, different places handle errors in different ways, some only log the error, some only emit the signal.

The best solutions would IMHO be to do both, as it allows the end user to choose how to react to errors:

• connect to errorOccurred() and handle the problem in user code
• enable the logging to see errors in a log
• ignore errors by disabling logging and not connecting to the errorOccurred() signal