Returned error code isn't correct

[imay]

Describe the bug
When a query is executed timeout, some other error code is returned.

To Reproduce
Steps to reproduce the behavior:

1. use xxx; xxx is a non-existed db, this will return with error code ERROR 1049 (42000)
2. execute a timeout query, this will return with error code ERROR 1049 (42000) other than ERROR 1064 (HY000) which is supposed to be.

This is because we don't reset ErrorCode in QueryState. And may be the cause of #736