core

The deliver service is currently quite ambiguous about the error
condition when the channel returned by Errored() closes.  This is likely
because this message could be returned by a peer or orderer.  However,
in reality, the only time this condition ever occurs is because of a
consenter error.  In the interest of serviceability, this CR makes the
error message more specific, making a note that we should handle
generalizing it if the peer ever implements this function.

Change-Id: I5c2dfb8fa4ba32c34660b69b5facf90291ee17f3
Signed-off-by: Jason Yellick <jyellick@us.ibm.com>