Fixes for DSOffice

- Added error messages (exceptions) for Excel nodes

- Removed "Microsoft" category which was showing under the "DSOffice" category (fix for [MAGN-2456](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-2456))

- Changed "Excel.AddExcelWorksheetToWorkbook" to return the worksheet instead of returning the workbook (fix for [MAGN-2458](http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-2458))