FCPClientReferenceImplementation.stop(): Catch newly thrown IOException

Was added to fcp_Unsubscribe() recently.
FCPClientReferenceImplementation.stop(): Indent, fix line length by
rephrasing
FCPClientReferenceImplementation.FCPMessageReceiver: Adapt to new
interface

- The interface was changed by a previous commit already, but the
function which implements it was not. handlePluginFCPMessage() is the
proper fcp-plugin-rewrite replacement for the previous function (which
was from the FredPluginTalker interface which is deprecated by the new
interface)
- The following commits will adapt the function body