FCPClientReferenceImplementation: Document "SubscriptionExistsAlready"
error handling

Should have been in the previous commit, sorry.
FCPClientReferenceImplementation.stop(): Fix InterruptedException
handling

- Thread.interrupted() is a function which is easily mixed up with
Thread.interrupt(): interrupted() queries the interrupted status,
interrupt() sets it to true. This mixup did happen in the old code, so
it is now changed to interrupt().
- Also, explanation of why we handle it like that is added
- If you do not unterstand the commit from the above and the added
documentation, I suggest you read this, its a very good overview of how
to properly deal with InterruptedException:
http://www.ibm.com/developerworks/java/library/j-jtp05236/