DeltaManagerProxy causes troubles when emitting "error" events. about fluidframework

[arinwt]

This issue was discovered in 0.9, before DeltaManagerProxy was set to use the EventForwarder base class (introduced in this PR). This should be mostly resolved in 0.10+, because the EventForwarder does not subscribe to the source emitter events until someone listens to the forwarder for that event.

Problem (in 0.9):

1. someone meaningfully listens to DeltaManager error event.
2. DeltaManagerProxy listens to DeltaManager error event.
3. nobody listens to DeltaManagerProxy error event.
4. DeltaManager raises error event, calling all listeners.
5. DeltaManagerProxy throws error because it has no error listeners.

Same flow with EventForwarder, no problem (in 0.10):

1. someone meaningfully listens to DeltaManager error event.
2. nobody listens to DeltaManagerProxy error event.
3. because of (2), DeltaManagerProxy does not listen to DeltaManager error event.
4. DeltaManager raises error event, calling all listeners.

But it could still happen with an unsubscribe (in 0.10):

1. someone meaningfully listens to DeltaManager error event.
2. someone listens to DeltaManagerProxy error event.
3. because of (2), DeltaManagerProxy listens to DeltaManager error event.
4. someone removes their only listener to DeltaManagerProxy error event.
5. DeltaManager raises error event, calling all listeners.
6. DeltaManagerProxy throws error because it has no error listeners.

I doubt this occurs in practice, because the proxy will likely be disposed which will disconnect it from the source emitter, but should still be fixed. When all listeners to a single event are removed from the proxy, it should stop listening to the source emitter for that event.