Version: EPP 4.0 RC.
When querying for a worksheet with an invalid index position the current error is unhelpful. It would be nice if the error explained the problem.
__Eample:__
```
using (var excelFile = new ExcelPackage(fi))
{
ExcelWorksheet worksheet = excelFile.Workbook.Worksheets[0];
}
```
Current error thrown:
```
Test 'TestCanReadImportFile' failed: System.Collections.Generic.KeyNotFoundException : The given key was not present in the dictionary.
at System.Collections.Generic.Dictionary`2.get_Item(TKey key)
at OfficeOpenXml.ExcelWorksheets.get_Item(Int32 PositionID)
ImportExport\ImportHallisTests.cs(31,0): at RAMS.Tests.ImportExport.ImportHallisTests.CanReadImportFile()
```
__Expected error:__
Something explaining that an invalid position was not found.
Comments: Fixed in changeset b0be4ad76772