Fix: check HTTP status in history.js fetch calls (#6282)

[JackieLin999]

Fixes #6282

Added HTTP status checking to the fetch calls in loadFirstChangesets
and loadMoreChangesets so that error responses (e.g. 504 gateway timeout)
are handled gracefully instead of being rendered as HTML in the history panel.

[tomhughes]

That's an awful lot of code just to check for load errors - was an AI perhaps involved in writing this?

I see there's also a lot of HTML being generated in javascript which we prefer to avoid where we can.

[HolgerJeromin]

Here a short path would be

.then(response => {
    if(!response.ok) {
        throw new Error("Request failed. Got http code: " + response.status + " " + response.statusText);
    }
    return response.text();
})

and a slim catch handler.