An escaped diagnostic says which file and which line, not (_)

Three suite runs left "Fatal error: exception Flan.Loc.Error(_)" on stderr:
OCaml's default handler knows nothing about the diag record, so a process
that does not catch prints a constructor name and none of the message. The
trigger was a corpus file the test binaries check directly and nothing there
wraps. Registering a Printexc printer changes no control flow and costs the
drivers that do catch nothing; it only makes the corpse legible.

lib/loc.ml

@@ -359,8 +359,8 @@ let report (d : diag) =
     it found. *)
 let report_all (ds : diag list) =
   (* Source order, with the placeless ones last. A diagnostic the checker
-     raised against [unknown] — a wrong [main] signature is the one that
-     happens — has line 0, and sorting on the number alone would put it at the
+     raised against [unknown] — a rule about a declaration that is not in this
+     file to point at — has line 0, and sorting on the number alone would put it at the
      top of the list, above every error that can actually be clicked. It is a
      real error and it is not anywhere, so it goes after the ones that are. *)
   let placed (d : diag) = d.dloc.line > 0 in
@@ -376,3 +376,24 @@ let report_all (ds : diag list) =
   let n = List.length ds in
   String.concat "\n" (List.map report ds)
   ^ Printf.sprintf "\n%d error%s" n (if n = 1 then "" else "s")
+
+(* The last line of defence, and the one nobody plans to reach. Every driver in
+   this tree catches [Error] and [Errors] and prints [report]; a process that
+   does not — a test binary walking the corpus, a tool written in an afternoon —
+   dies through OCaml's default handler, and the default handler knows nothing
+   about this record. What it prints is [Fatal error: exception
+   Flan.Loc.Error(_)]: the name of a constructor, an underscore, and not one
+   word of the diagnostic that was carefully built to say what was wrong and
+   where. That is how three suite runs came to leave a message-less fatal on
+   stderr while reporting that they had passed.
+
+   Registering a printer costs nothing and cannot change control flow — the
+   exception still propagates and still kills whatever did not catch it. All it
+   changes is that the corpse says which file and which line, which is the
+   whole of what the diagnostic was for. Drivers that do catch are unaffected:
+   they never ask [Printexc] anything. *)
+let () =
+  Printexc.register_printer (function
+    | Error d -> Some (report d)
+    | Errors ds -> Some (report_all ds)
+    | _ -> None)