'web proof just broke' email gives no clues about how to fix (UI/doc issue)

When a web proof breaks, from Keybase's point of view, it sends a message

your previously-proven web identity XXX just broke.
We've been checking it repeatedly,
and it's not working from our perspective.

Can I point out that 'it's not working' is _the worst bug report ever_? You wouldn't accept that in a github bug report.

I suggest that the message should indicate:

1. _what's_ not working (ie, what actual error have you seen?)
2. where to go to debug this (ie, a pointer to documentation, perhaps including a checklist of things to examine)

In my particular case, it was because I'd changed the certificates, and was sending out just a certificate, and not the full chain. That's easily fixed, but I only worked out _what_ needed fixed by searching for bug reports, and finding (very useful) #2914.

The email does include two suggestions:

* If you deleted a public proof, remember that proofs *must* stay public so anyone can trust you without trusting the Keybase.io server.
* If your identity has changed, you should revoke or replace the proof with the Keybase app.

But these are both rather generic suggestions, neither of which was relevant to my case (and those below), and neither of which says what's _actually_ wrong, so neither is as useful as was doubtless intended.

Related:

* Issue #3773 is a general one about documentation, but which covers this.
* Issue #2878 is fundamentally caused by this issue, but manifesting as difficulty re-proving.
* Issue #1711 is a different underlying cause of the same error.

These are all at least partly spurious bug reports, which create noise for you, and frustration for users; I stopped searching when I found these, so there may be other related ones. They can potentially all be closed by fixing this one.

Start by locating the web-proof failure email and the related documentation referenced by issues #3773, #2878, and #1711. Review the reported certificate-chain case and other failure causes, then update the message so it identifies the observed error and points to actionable debugging guidance.

Written by the indexing model from the issue text.