improve index support for mssql dialect

  - bring addIndexQuery in line with abstract dialect
  - use an improved method for showing indices with sp_helpindex
  - actually use handleShowIndexQuery (bringing us closer to the
    abstract dialect for formatResults)
  - temporarily disable tests for a FULLTEXT index
fixed a few more failing cases

  - missed a case for addLimitAndOffset for subqueries with a single model
  - formatResults should return undefined if there is no data
  - there are a few possible error messages in an invalid connection scenario