SubscribeClient should not panic

[ggwpez]

Location

[wire] Localbus.SubscribeClient the close(recv.exists) statement.

Problem

Code panics when it is called twice with the same address.
AFAIK the local bus was only intended for testing and not production usage, so some edge-cases which are not needed for testing were omitted.

Proposal

Return an error instead and add a test+race test.