ODBC: Native codes are lost when calling SQLGetDiagRec repeatedly

When SQLGetDiagRec is called repeatedly in qWarnODBCHandle, in order to obtain all available diagnostics, the message texts are concatenated and all of them returned, but the native code is overwritten for every call.

This means that if there a severe error first followed by less important warnings later, querying lastError().number() will only contain really the last of the errors and the text will contain all texts concatenated.

I suggest that all native errors are passed up somehow.