Fix UnboundLocalError in mssql during isolation level grab

Fix UnboundLocalError in mssql during isolation level grab

Fixed issue within the SQL Server dialect under Python 3 where when running
against a non-standard SQL server database that does not contain either the
"sys.dm_exec_sessions" or "sys.dm_pdw_nodes_exec_sessions" views, leading
to a failure to fetch the isolation level, the error raise would fail due
to an UnboundLocalError.

Fixes: #4273
Co-authored-by: wikiped <wikiped@yandex.ru>
Change-Id: I39877c1f65f9cf8602fb1dceaf03072357759564