Using Do/Catch in Swift

I am working on an app and want to get data back from a function. However sometimes data is missing or is different from the kind of that I want to retrieve. I am new to Swift and I can't find a way to write a function that does a little bit of processing and returns this data. When this data is missing, the function should give back a string "Not Found". Like this:

func processData(data:String) {
    do {
        //processing
        var result = processedData
    } catch {
        var result = "Not Found"
    }

    return result
}

It would be very nice if somebody could help me.