rpclib: Allow ConnectionClosed in arti_socket_closed test

This test is supposed to check the case where we launch a bunch of requests but the socket is dropped before we get a response. Different operating systems handle 'drop' a bit differently, and schedule threads differently, so we shouldn't be surprised at slightly different errors here.

In particular, OSX is more prone to deliver the close as a proper "close" rather than an EPIPE or such, which previously broke this test.

Closes #1510 (closed).

(Yes, I am on vacation, but sometimes one wants to hack in the train.)