add flight err back (#895)

conn.go

@@ -711,6 +711,7 @@ func (c *Conn) prepareStatement(ctx context.Context, stmt string, tracer Tracer)
 
 	framer, err := c.exec(ctx, prep, tracer)
 	if err != nil {
+		flight.err = err
 		flight.wg.Done()
 		c.session.stmtsLRU.remove(stmtCacheKey)
 		return nil, err