Change error type returned to an actual error type

#### Description:

When something crashes in code executed, might be due to anything really, you generally get `ExcelErrorNull` returned from the toolkit. This type of error is _not_ recognised as an error by the native `=IFERROR(` command in Excel.

This can be quite annoying when you want to set up a sheet to be ready to receive data from an adapter or similar, where a simple `=IFERROR( do some stuff, "")` can be a quite nice way to keep the sheet looking clean without to much hassle.

Would suggest a `#VALUE` or similar to be returned instead (not sure if there is like `#NULL' or similar that would be an even better fit), basically, something picked up by Excel as being an error.

https://github.com/BHoM/Excel_Toolkit/blob/master/Excel_UI/Addin/AddIn_Converts.cs seem to be the main file handling this from my quick search! :)