FCPInterface.handleSubscribe(): Store the error type in
FCPPluginMessage.errorCode

- This replaces the old mechanism of putting the error in the
"Description" field in params.
- errorMessage is not added because as said in the comments, the
Subscribe FCP function is not typically used by UI. (errorMessage shall
typically be localized, so it would be a bit of work to add the
translation strings).
- We don't break any existing client applications with removing the
"Description" field because the whole subscriptions code has not been
released in an official WOT build yet. So clients should not be using it
yet.
- Also fixes the existing parsers of WOT, namely the
FCPClientReferenceImplementation and the unit test
SubscriptionManagerFCPTest to use the new place for the error code.
FCPInterface.handleSubscribe(): Add some missing JavaDoc <br>

Doesn't add them to the whole of the JavaDoc, parts of it are subject to
change anyway.
FCPInterface.handlePluginFCPMessage(): Fully document error messages

- Fix documentation of SubscriptionExistsAlready message to mention that
the identifier will be the same as in the original Subscribe message.
- Document which errorCode and other stuff is used if the user indicated
failure of processing the synchronization message. I've validated this
with a test run. This also fixes the fact that the documentation about
that error message was not int he "Errors" section, and that it looked
like the error message would be of type "Message" = "Subscribed", which
it is not.