Features: #304 — member routes name a herdr failure

11-Features.md

@@ -3666,3 +3666,28 @@ its guard checked `content == null`, not blank, so whitespace went through. Movi
 handler as an uncaught exception. The tool's own guard was widened to `isBlank` for that reason, and
 `FleetMcpTest.replyWithBlankContentIsACleanToolErrorNotAnUncaughtException` pins it. If you ever move
 the check again, check the handler between the door and the service, not only the two ends.
+
+## The member REST routes name a herdr failure instead of returning a bare 500
+
+**What.** `POST /members` and `DELETE /members/{paneId}` were the last two routes in `FleetApp`
+with no `catch (HerdrException)`. `FleetApp` registers no Javalin exception mapper, so the exception
+escaped as the default 500 with the body `Server Error` — no herdr code, no herdr message. Both now
+go through the same `herdrError` mapper every other herdr-calling route uses: 404 when herdr says
+the target is gone, 502 otherwise.
+
+**On.** Always on.
+
+**Why it exists.** `fleet_spawn` and `fleet_stop` already caught the same exception and reported a
+named error, so the two doors disagreed about the same failure — the shape #297 exists to remove.
+The stop route is the one that mattered: `SessionManager.release` deregisters the session, notifies
+the release listener and preserves a dirty worktree *before* it calls `launcher.stop`, so a throw
+from that stop arrives after the teardown the caller asked for has already happened. A 500 told the
+caller to retry and gave it nothing to reason about.
+
+**One thing to know for maintenance.** An already-gone pane is **not** one of these failures.
+`HerdrPeerLauncher.stop` treats a `*_not_found` from `pane.close` as success, so a double stop still
+returns 204, and `stopToleratesAnAlreadyGonePane` pins that. What propagates is any other herdr
+failure — a transport error, or a code like `herdr_busy`. No new tests were added: two tests already
+covered these paths and asserted 500, and neither was about the status code (one guards that a
+failed spawn still closes its tab, the other that a failed teardown is not reported as 204). Their
+expectations moved to 502 and gained a body check.