Fix the bake scan report's unsound absence-as-signal claim

Gate the summary on any unanswered key and on SetProjExtState's return, print one line per scanned key, and make the undo block stack RAII.

src/shell/instrument/instrument_bake.cpp

@@ -196,32 +196,34 @@ BakeChainResult runBake(ReaSamplerProcessor& processor) {
 
     // What the key holds now is the only evidence this side gets, and each of the five
     // non-answers is a different thing to go fix — collapsing them into one sentence is
-    // what made a stale install indistinguishable from a refusal.
+    // what made a stale install indistinguishable from a refusal. The three that are about
+    // the KEY name it, because the landing prints one console line per key it scanned and
+    // the key is what correlates the two in a multi-instance session.
     const wire::BakeAnswer answer =
         wire::classifyBakeAnswer(bridge.readReasamplerExtState(key), request);
     switch (answer.kind) {
         case wire::BakeAnswerKind::Answered:
             break;
         case wire::BakeAnswerKind::Unanswered:
-            return fail(
-                "the ReaSampler extension did not answer -- the request key still holds "
-                "this exact request, untouched. Check the REAPER console: if the landing "
-                "action printed a scan report just now, it DID run and that line says what "
-                "it saw; if the console is silent, the action never ran at all");
+            return fail("the ReaSampler extension did not answer " + key +
+                        " -- that key still holds this exact request, untouched. The "
+                        "landing action prints one REAPER console line per key it "
+                        "scanned; look for that key name there");
         case wire::BakeAnswerKind::Undecodable:
             return fail(
                 "the extension answered in a format this plugin does not read -- the "
                 "extension and ReaSampler 9000 are from different builds");
         case wire::BakeAnswerKind::Cleared:
-            return fail(
-                "the bake key came back empty -- either the request was cleared before "
-                "an answer was written, or this build could not read whatever was there. "
-                "The landing action's console scan report, if one appeared, names which");
+            return fail("the bake key " + key +
+                        " came back empty -- either the request was cleared before an "
+                        "answer was written, or this build could not read whatever was "
+                        "there. Look for that key name in the landing action's console "
+                        "lines");
         case wire::BakeAnswerKind::ForeignRequest:
-            return fail(
-                "the bake key held a different pending request instead of an answer -- "
-                "unexpected given the bake's single-threaded call flow; if this recurs, "
-                "note the exact steps and file it");
+            return fail("the bake key " + key +
+                        " held a different pending request instead of an answer -- "
+                        "unexpected given the bake's single-threaded call flow; if this "
+                        "recurs, note the exact steps and file it");
         case wire::BakeAnswerKind::ForeignOutcome:
             return fail("the extension answered a different bake request");
     }