Make it easier for users & admins to debug download issues

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**What problem will this feature request solve?**

When a CSV/XLSX/JSON download fails, people usually only see “Download error” or “Could not download the file.” The real reason (query error, permissions, timeout, incomplete file, too many characters in a cell for Excel) is in the browser network tab or server logs.

**Describe the solution you'd like.**
There may be other options but I'm thinking of 2 complementary pieces:

1. In the download status UI: When the request fails before streaming starts, show the server error (error / message from the response), not a generic string. A “Show error details” dialog like we already have for uploads comes to mind.

2. For mid-stream failures maybe we can use the existing query_execution row (error + download context) so an admin can see that an export failed and why via Usage Analytics. That could be a small addition to "Erroring questions" (or surface somewhere that makes sense in the upcoming "Monitor" tool)?

**How does this feature request impact you?**
Users often have to reach out for help understanding what's going on. We end up walking them through DevTools or logs for a message that, if surfac,ed could be useful to an admin in many cases. At minimum it makes it easier for them to find the diagnostics they need to ask about and shortens the support loop.

Start with the download status UI and compare its failure handling with the existing upload error-details dialog. Then trace the query_execution error and download context into Usage Analytics, including Erroring questions and the upcoming Monitor tool; done means users or admins can see useful diagnostics for both pre-stream and mid-stream failures.

Written by the indexing model from the issue text.