RawSheetImporter's ImportRow should wrap page.GetCell in the try block

[doug-w]

I was implementing my own RawSheetImporter and ran into an issue where only about half of the pages in the sheet were being loaded. It turns out that GetCell() was throwing but because it's wrapped in a Task called from an Async MenuItem, nothing actually caught the exception.

I think the GetCell call in ImportRow should be part of the same try/catch that propertyMap.SetValue() is.

[cathei]

By convention, GetCell should not throw and just return null if cell does not exist / out of bounds. Exception from GetCell cannot be caused by data and if it happens that means it's bug in BakingSheet or importer. Will add comment to make this clear.