Client receipt of MSG_CHANNEL_FAILURE doesn't surface usefully

[bitprophet]

At least, during situations like those described in the middle of #398:

• Implement the env channel message for setting env vars, but configured to expect a reply from the server & thus also set up to wait around for said reply.
  • The live implementation does not do this because ssh doesn't either; but tweaking that code is an easy way to trigger the bug I'm describing here.
• Server appears to send MSG_CHANNEL_FAILURE if AcceptEnv rejects a given env set
• Our msg receipt logic isn't dealing with this gracefully; the _wait_for_event loop at the end of the message-send method dies because the channel gets closed, with SSHException('Channel closed.') (actually, wrapped within another SSHException).
• What should happen instead, presumably, is for something - I'd guess the MSG_CHANNEL_FAILURE handler - to surface a more useful error message, so we see that message instead of the default "well, I'm closed, but I never exited with a success message, so I'll just make up my own error" (here)