Creates a channel with an optional buffer, an optional transducer (like (map f), (filter p) etc or a composition thereof), and an optional exception-handler. If buf-or-n is a number, will create and use a fixed buffer of that size. If a transducer is supplied a buffer must be specified. ex-handler must be a fn of one argument - if an exception occurs during transformation it will be called with the Throwable as an argument, and any non-nil return value will be placed in the channel.