SubscriptionManager: Add FIXME to handle InterruptedException

- FCPInterface was recently changed to throw new Exception types,
especially InterruptedException. Since the SubscriptionManager has
generic catch blocks around things such as sending notifications to
clients, it will swallow them. Instead, it should shutdown its thread.
FCPInterface: Extract function throwIfClientReplyIndicatesError()

- Will be used to fix existing callers of sendSynchronous() to pass the
error to SubscriptionManager
FCPInterface: Fix ignoring of FCPPluginMessage.success==false with
sendSynchronous()

- Sorry. To prevent this from happening again, I've improved the JavaDoc
of sendSynchronous(). See f4cd960139c1bedf00bdb10b88f6557b23fca3f4