validate property access to caught errors

[OlafConijn]

typescript errors that get thrown can be caught. in typescript the type that gets caught is an instance of the error. in ASL the type that gets "caught" is a structure that looks like {Error?: string, Cause?: string}.

would be good to both:

• create a asl.error(error, cause) function that return an instance of an AslError
• have the asl.error(error, cause) function also ensure the error is set to the name of the Node Error. this will ensure errors in lambda functions are correctly caught
• validate member access of the caught variable. e.g. error.something would throw even though that field exists in typescript.