codegen: CallError out-param instead of throwing wrappers

Per review, the generated sync wrappers expose the error channel as an
optional trailing parameter — add(a, b, &err) — rather than throwing:
explicit, stateless, works on temporaries, and existing call sites
compile unchanged (they keep default-on-failure, now with a qWarning so
failures are visible in the module log). The dispatch catch-all from the
previous commit stays: it contains author exceptions, it doesn't
introduce any.

cpp-generator/legacy/main.cpp

@@ -612,10 +612,9 @@ static int generateProviderDispatch(const QString& headerPath, const QString& ou
         methodsByName[m.name].append(&m);
     }
 
-    // The dispatch body is wrapped in a catch-all: anything the author's code
-    // (or a generated typed wrapper — see logos::LogosCallError) lets escape
-    // becomes an ordinary method failure (invalid QVariant) instead of an
-    // exception unwinding through Qt event dispatch and killing the module
+    // The dispatch body is wrapped in a catch-all: any exception the author's
+    // code lets escape becomes an ordinary method failure (invalid QVariant)
+    // instead of unwinding through Qt event dispatch and killing the module
     // process.
     s << "QVariant " << className << "::callMethod(const QString& methodName, const QVariantList& args)\n";
     s << "{\n";