FCPClientReferenceImplementation...handlePluginFCPMessage(): Adapt
to new error signaling in plugin-fcp-rewrite

- The old pre plugin-fcp-rewrite code was using the feature of
branch PluginTalker-sync-send which allowed it to throw out a
RuntimeException  to cause that RuntimeException to be passed to the
sender of the  original message. This mechanism has been replaced by
returning a FCPPluginMessage object which was constructed as a reply to
contain success==false, which we can do by using
FCPPluginMessage.constructErrorReply().
- Also, we must only return such a reply if the original message was a
reply already - replies to replies are not allowed by JavaDoc of
FredPluginFCPMessageHandler.
- While I was it, the documetnation about the "dangling
subscription" was amended to stress that this means the *remote* code
at WOT: We're sending a reply to the remote side so it can terminate
the subscription, so the statement that it is remote should be added to
the documentation
FCPClientReferenceImplementation...handlePluginFCPMessage(): Adapt
to new function signature

- The logging string is put into a variable since a following commit
will need it
- Log the full message instead of only the SimpleFieldSet since
FCPPluginMessage contains a toString().
FCPClientReferenceImplementation...handlePluginFCPMessage(): Add missing
return

- The !message.isReplyMessage() check is there because we must not reply
to messages which were a reply already.
- The error signaling by returning an error reply here is only a service
to the  remote side. We must also return if we are not allowed to return
a reply - if we are in this state of error we are done.
- Returning null is allowed, see the parent interface's JavaDoc.