Improve handling of bulk build reports

[bsiegert]

The red error bar saying "try recreating the index" is a pretty bad UI.

• store the last fetch error in the database and surface it
• use a custom queue to increase the time between fetches
• for extra points: make the reindex show the status interactively.

[bsiegert]

Design sketch:

• [ ] JSON endpoint for calling ingest.FetchReport, which runs in the foreground instead of in the background as before.
• [ ] Detect 0 results in the build status page and offer a button for triggering a re-index
• [ ] When clicked, XHR request, wait for it to come back, then show the status directly on the same page.