[openstreetmap/openstreetmap-website] Image download with user feedback (PR #6192)

[mmd]

mmd-osm left a comment (openstreetmap/openstreetmap-website#6192)

Currently the export script returns errors as text/html, which Turbo happily picks up to replace the osm.org page. In this case, the user has to reload the page. Browser back button would take them to a random place, which is quite annoying.

Maybe it's better for the export script to return error messages as plain text. Turbo ignores this response, we can display the alert popup and nothing else happens.

As an alternative, https://discuss.hotwired.dev/t/finally-understanding-turbo-stream/1964/8 talks about some funny stuff we could do with Turbo Streams, though it seems a bit over-engineered for this use case.

leaflet.share.js:

```js
      } else {
        let detailMessage;
        try {
          detailMessage = event?.detail?.error?.message;
          if (!detailMessage) {
            detailMessage = await event.detail.fetchResponse.responseText;
          }
        } catch {
          detailMessage = "(unknown)";
        }
        // eslint-disable-next-line no-alert
        alert(OSM.i18n.t("javascripts.share.export_failed", { reason: detailMessage }));
      }
```
export.erb:

```python
# Routine to report an error
def output_error(message, status = "400 Bad Request"):
  print("Status: %s" % status)
  output_headers("text/plain")
  print(message)
```

Result:

<img width="414" height="138" alt="image" src="https://github.com/user-attachments/assets/1b4c99b3-3cdd-4c3a-a1f9-b62ef035595d" />


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