Excel error handling

[cyberop5]

Enforce DataRange/Dataset when creating ExcelFileDefinitions.
Current behavior: Throws a null reference exception because reader.DataRange is null.
Change:

1. Check that the WithDataset address isn't null.
2. Throw a more descriptive null reference exception when calling GetExcelReader.
3. Throw a more helpful null reference exception when trying to read a row without a DataRange.