PR #13135: a failed conversation read says so, and Try again recovers

PR #13135 at 6fb5657f7c. Local dev server against the stage table (read-only). The failure is a local-only, file-gated throw at the top of the conversation read. It was never committed and was reverted after the run. Only the error card is shown, with no names.

Before (main): the prospect page's box

Before: We couldn't load this prospect. The record is there — something in it didn't render.

This names the wrong record (the prospect) and the wrong cause ("didn't render"), and there is no way to retry.

After: the conversation's own boundary

After: We couldn't load this conversation. Try again. Back to prospect.

Checked in the page text: exactly "We couldn't load this conversation.", "Try again" and "Back to prospect". It does not contain "didn't render" or "this prospect".

Try again re-reads

With the fault still on, Try again showed the same error again: the read ran again and failed again. After the fault was cleared, Try again rendered the thread (10 messages) on the same URL. The thread itself is not pictured because it shows stage names.