Serialize: Report the first bad section, and start with no title

SetError overwrote the first bad section with whichever section a later
error came from, and an error before any section read an uninitialized
curTitle_.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

Common/Serialize/Serializer.cpp

@@ -125,7 +125,9 @@ void PointerWrap::SetError(Error error_) {
 		// and also not logspam like MEASURE will do in an error case.
 		mode = PointerWrap::MODE_NOOP;
 		// Also, remember the bad section.
-		firstBadSectionTitle_ = curTitle_;
+		if (!firstBadSectionTitle_) {
+			firstBadSectionTitle_ = curTitle_;
+		}
 	}
 }