Return an err if sending to a nil BasicOutStream.

[j0sh]

Prevents a segfault further down in the same method.

The segfault was reproducible under the following scenario on devenv:

0. Begin broadcasting (also act as bootnode for transcoder)
1. Transcoding begins.
2. Kill transcoder (ctrl-C in this case; not sure if this was a 'clean' kill as far as TCP teardowns go).
3. Broadcaster / bootnode attempts to reconnect to transcoder
4. Reconnect fails
5. Broadcaster tries to send a data message anyway
6. 💥

By returning ErrOutStream here, the listener also gets removed and the broadcaster no longer attempts to send data.

[ericxtang]

Good catch. 🚢