More defensive approach when it comes to saving files

[wouterfennis]

When you currently save a spreadsheets that already exists on disk the program will fail.
We need more defensive programming to prevent this.
The https://github.com/wouterfennis/SpreadsheetWriter needs to be more robust and give a clear message if the saving went successful or that an error did occur.
Cifra will have to handle the situation where a file can't be saved.
Maybe ask for an other name or try again.