Simplest way to throw an error/exception with a custom message in Swift 2?

The simplest approach is probably to define one custom enum with just one case that has a String attached to it:

enum MyError: ErrorType {
    case runtimeError(String)
}

Or, as of Swift 4:

enum MyError: Error {
    case runtimeError(String)
}

Example usage would be something like:

func someFunction() throws {
    throw MyError.runtimeError("some message")
}
do {
    try someFunction()
} catch MyError.runtimeError(let errorMessage) {
    print(errorMessage)
}

If you wish to use existing Error types, the most general one would be an NSError, and you could make a factory method to create and throw one with a custom message.

The simplest way is to make String conform to Error:

extension String: Error {}

Then you can just throw a string:

throw "Some Error"

To make the string itself be the localizedString of the error you can instead extend LocalizedError:

extension String: LocalizedError {
    public var errorDescription: String? { return self }
}

@nick-keets's solution is most elegant, but it did break down for me in test target with the following compile time error:

Redundant conformance of 'String' to protocol 'Error'

Here's another approach:

struct RuntimeError: Error {
    let message: String

    init(_ message: String) {
        self.message = message
    }

    public var localizedDescription: String {
        return message
    }
}

And to use:

throw RuntimeError("Error message.")