Continually produce exceptions for InputStreams

[wraithm]

Right now, the heartbeat checker on an input stream will die on the first heartbeat. Instead, the semantics should be to continually produce exceptions in the case that heartbeats continue not to show up. It should be up to the user of this library to decide how to handle these exceptions.

[wraithm]

Or maybe have a higher order function argument to input stream heartbeat checking that runs on the first missed heartbeat, then throw the exception and shut down on the end of the grace period