channeldb: fix channel corruption bug upon channel closure

This commit fixes a bug which would previously lead to corruption of
the channel state when a node had one or more channels open and one of
them was closed either forcibly or cooperatively. The source of the bug
itself as a typo: rather than using the construed `deliveryKey`
variable to fetch/put/delete the delivery scripts, `deliveryScriptsKey`
(the key prefix itself) as used. This bug would cause the database to
be unable to read _any_ channel from the database after one was
deleted, as each channel would actually be reading/writing-to the
_exact same_ delivery script.

The fix for the bug itself is simple: eliminate the typo.

channeldb/db.go

@@ -276,7 +276,7 @@ func (d *DB) fetchNodeChannels(openChanBucket,
 			nodeChanBucket, chanID)
 		if err != nil {
 			return fmt.Errorf("unable to read channel data for "+
-				"chan_point=%v", chanID)
+				"chan_point=%v: %v", chanID, err)
 		}
 		oChannel.Db = d