Emit sl-error in sl-icon for failing requests

[ibirrer]

Listening to the sl-error event of sl-icon is not triggered if the request to the svg fails. Example

<sl-icon id="triangle" name="exclamation-triangle"></sl-icon>
<sl-icon id="missing"  name="missing"></sl-icon>

<script>
  // works fine
  document.getElementById("triangle").addEventListener("sl-load", () => console.log("loaded"))
  // never triggered
  document.getElementById("missing").addEventListener("sl-error", (e) => console.log("error", e))
</script>

This PR fixes this by emitting the event if file.ok is false.

[vercel]

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