Sybase/MSSQL: Fix for repeated EED messages

This pull request fixes repeated EED server messages causing error handling to break subsequent queries. An error like this can be provoked by issuing a query as follows:

```sql
select not_the_table_name.field1, not_the_table_name.field2 from table_name
```

This pull request adds an integration test to verify this. If this test is run with the implementation existing on the master branch, the effect can nicely be seen:

```
F unittest.TestError(test= net.xp_framework.unittest.rdbms.integration.SybaseIntegrationTest::repeated_extend_errors, time= 0.027 seconds) {
  Exception rdbms.SQLStatementFailedException (errorcode 0: Unexpected token 0x5F (number 0)) {
    select 1
  }
```
Idea:

```php
$cause= null;
while (array_shift($this->messages)) {     // ...or maybe: array_shift($this->messages['errors']))?
  $cause= new TdsProtocolException(..., $cause);
}
throw $cause;
```