lm: Silence no return value warning

src/core/hle/service/lm/manager.cpp

@@ -86,7 +86,8 @@ std::string FormatField(Field type, const std::vector<u8>& data) {
         return Common::StringFromFixedZeroTerminatedBuffer(
             reinterpret_cast<const char*>(data.data()), data.size());
     default:
-        UNIMPLEMENTED();
+        UNIMPLEMENTED_MSG("Unimplemented field type={}", type);
+        return "";
     }
 }