FCPPluginClient.send(): Fix logging of messsages for which the handler
threw

- Log the message which caused the handler to throw, not the reply
(which will be null anyway if the handler threw)
- Implement the missing toString() of FCPPluginMessage
FredPluginFCPMessageHandler: Document how to transport error
descriptions

- FCPPluginMessage nowadays has fields errorCode and errorMessage so we
shall tell the ServerSideMessageHandler / ClientSideMessageHandler to
use them.