Apparently when the fetch fails, it triggers an `fetch:error` event[1], which can (probably) be handled somewhere. No idea about formatting the response, though.
I think it's quite natural for there to be large gaps in functionality provided: it looks like a rather young and unpopular project, so the additional code needed to handle more situations from within the language still needs to get written. jQuery also didn't have a whole universe of plugins from day 1, but was useful enough anyway. Not saying this project has a chance to become another jquery, just that it too can be useful even in a far-from-complete state.
[1] https://github.com/bigskysoftware/_hyperscript/blob/master/s...