A function that enables you to easily wrap throwing APIs, to provide a custom error

Perform.swift

/**
 *  Perform a throwing expression, and throw a custom error in case the expression threw
 *
 *  - parameter expression: The expression to execute
 *  - parameter error: The custom error to throw instead of the expression's error
 *  - throws: The given error
 *  - returns: The return value of the given expression
 */
func perform<T>(_ expression: @autoclosure () throws -> T, orThrow errorExpression: @autoclosure () -> Error) throws -> T {
    do {
        return try expression()
    } catch {
        throw errorExpression()
    }
}

You should probably also mark the error parameter as an autoclosure, letting it be created only when an error happens.

I had to go with

catch _ { 
    throw error
}

as catch was using the error thrown on try rather than the error passed in the function