Change the interface of Parallel from returning a slice of interface{}s to calling a callback function with the correct parameters, as well as a last error parameter
Make the done function of Parallel take a proper error parameter. Thanks Rémy Oudompheng! (https://groups.google.com/forum/#!topic/golang-nuts/WOUft8EZQ1I)
Runtime type check that done-func final arg is an error; Make runtime type check error strings consts