Server errors don't display when using a same-origin endpoint URL.

[lucas42]

Steps to Reproduce

1. Create a new Yasgui instance, where endpoint in the requestConfig provides a path on the same origin, rather than a full URL. For example:

import Yasgui from "@zazuko/yasgui";
import "@zazuko/yasgui/build/yasgui.min.css";

new Yasgui(document.getElementById("yasgui"), {
	requestConfig: { endpoint: "/sparql" },
	copyEndpointOnNewTab: false,
});

2. Make a request through Yasgui which returns a HTTP error (eg a 5xx caused by a query timeout).

Expected Behaviour

The body of the erroring HTTP response to be shown in the UI, eg:

Actual Behaviour

Nothing is show in the response section of the UI.

The following error is logged in the browser's console:

yasgui.min.js:2 Uncaught (in promise) TypeError: Failed to construct 'URL': Invalid URL
    at yasgui.min.js:2:983497
    at f (yasgui.min.js:2:780774)
    at Generator.<anonymous> (yasgui.min.js:2:782097)
    at Generator.next (yasgui.min.js:2:781184)
    at yasgui.min.js:2:971919
    at new Promise (<anonymous>)
    at yasgui.min.js:2:971664
    at yasgui.min.js:2:983337
    at t.<anonymous> (yasgui.min.js:2:136769)
    at f (yasgui.min.js:2:780774)

Workaround

Add window.location.origin to the start of the endpoint.

import Yasgui from "@zazuko/yasgui";
import "@zazuko/yasgui/build/yasgui.min.css";

new Yasgui(document.getElementById("yasgui"), {
	requestConfig: { endpoint: window.location.origin+"/sparql" },
	copyEndpointOnNewTab: false,
});

It's unclear from the documentation whether the endpoint parameter is intended to only work for absolute URLs, or if relative ones are acceptable too. If there's meant to be a requirement for absolute URLs, I think it'd be more developer-friendly to check for that early on.