ErrorTypeExtension.swift

extension ErrorType {
    var errorSummary: String {
        return ""
    }
}

extension NSError {
    var errorSummary: String {
        return localizedDescription
    }
}

let error: ErrorType = NSError(domain: "com.github.naoty.playground", code: 1000, userInfo: [NSLocalizedDescriptionKey: "Something wrong"])
print(error.errorSummary) //=> "\n"

FriendlyErrorType.swift

protocol FriendlyErrorType: ErrorType {
    var errorSummary: String { get }
}

extension NSError: FriendlyErrorType {
    var errorSummary: String {
        return localizedDescription
    }
}

let error: FriendlyErrorType = NSError(domain: "com.github.naoty.playground", code: 1000, userInfo: [NSLocalizedDescriptionKey: "Something wrong"])
print(error.errorSummary) //=> "Something wrong\n"