exit-specific streams fail if the client has no descriptor

If a particular stream requires a specific exit, and there is no descriptor for that exit, then the Tor client will fail immediately, sending STREAMSTATUS FAILED to the controller.

We would like to provide the controller an opportunity to provide Tor appropriate descriptor(s) for this stream on demand. Specifically, this means sending the controller a STREAMSTATUS NEW_CONNECT and allowing the controller to post descriptors. At this point, the controller can pass the task back to Tor or manage the stream as it so desires.

[Automatically added by flyspray2trac: Operating System: All]