Allow for custom error message during BARF recovery

[Folakuku]

Either directly pass a RecoveryMessage or pass a RecoveryHandler, a function that gives you access to the panic error, manipulate it as needed, maybe even send an alert somewhere and then return a response string that's sent as the server response.

Here's sample use case

        // create server
	if err := barf.Stark(barf.Augment{
		Logging:        barf.Allow(), // enable request logging
		Recovery:       barf.Allow(), // enable panic recovery so barf returns a 500 error instead of crashing
                RecoveryHandler: func (err error) string {
                        return err.String() 
                },
                RecoveryMessage: "Something went wrong",
	}); err != nil {
		barf.Logger().Error(err.Error())
		os.Exit(1)
	}