Various: Report errors encountered while doing channel I/O.

Various: Report errors encountered while doing channel I/O.

I'm amazed I let these get swallowed up between chan and endpt.  Oh,
well.

Minor interface change: endpt_join wants a channel description string,
but the source (which always does the joining anyway) has one to hand.
The channel structure has grown an error indicator which the endpoint
manager inspects when it's tearing down the connection.