A better approach would be:

// ErrorType Protocol.
enum ConversionError: ErrorType {
    case NotAnInt
}

// A function that "throws".
func stringToInteger(stringValue: String) throws -> Int {
    guard Int(stringValue) != nil else {
        throw ConversionError.NotAnInt
    }
    return Int(stringValue)!
}

// Input.
let inputString = readLine()!

// Logic.
do {
    try print(stringToInteger(inputString))
} catch {
    print("Bad String")
}

But even that is not accepted.

This exercise is broken for Swift.