fix SendRaw Failfast mode swallows error (#902)

SendRaw in Failfast mode always returned nil as the error even when
wrapSendRaw failed. Return the actual error to match the behavior
of Call's Failfast mode.

client/xclient.go

@@ -869,7 +869,7 @@ func (c *xClient) SendRaw(ctx context.Context, r *protocol.Message) (map[string]
 			}
 		}
 
-		return m, payload, nil
+		return m, payload, err
 	}
 }