return a known error code when max rows is exceeded

[bbeaudreault]

I chose negative numbers for the vitess error codes so they dont conflict with mysql, but could be easily swayed to choose something else if preferred. There are no pre-existing mysql error codes that really make sense for this.

We could also add another field to SQLError, if that would be preferred.

[sougou]

We should find out what error code mysql returns for the max rows setting and mimic that.

[bbeaudreault]

@sougou I scanned https://dev.mysql.com/doc/refman/5.7/en/error-messages-server.html and I didn't see any error codes that made sense. I don't think mysql sets a limit on this?

There is an ER_TOO_MANY_ROWS, but that is specifically for the case of select * from foo where a = (select b from bar), when the inner query returns more than 1 row.

[sougou]

LGTM
I think this is ok for now. But we have to keep an eye on it. It could become a problem later if some third party tools get confused by the error code.