Don't drop only partially supported GTFS-RT messages

Previously all messages containing unsupported fields like (alert or
vehicle) were ignored, even if they contained an understandable
trip_update.

This commit also changes the debug output to show the names of the
relevant fields to make it easier to debug GTFS-RT feeds without looking
at the MOTIS source code.

src/rt/gtfsrt_update.cc

@@ -270,33 +270,33 @@ statistics gtfsrt_update_msg(timetable const& tt,
   for (auto const& entity : msg.entity()) {
     if (entity.has_is_deleted() && entity.is_deleted()) {
       log(log_lvl::error, "rt.gtfs.unsupported",
-          "unsupported deleted (tag={}, id={})", tag, entity.id());
+          R"(unsupported "id_deleted" field (tag={}, id={}), skipping message)", tag, entity.id());
       ++stats.unsupported_deleted_;
       continue;
     } else if (entity.has_alert()) {
       log(log_lvl::error, "rt.gtfs.unsupported",
-          "unsupported alert (tag={}, id={})", tag, entity.id());
+          R"(ignoring unsupported "alert" field (tag={}, id={}))", tag, entity.id());
       ++stats.unsupported_alert_;
-      continue;
+      // no continue here so we don't skip as long as it also has a trip_update
     } else if (entity.has_vehicle()) {
       log(log_lvl::error, "rt.gtfs.unsupported",
-          "unsupported vehicle (tag={}, id={})", tag, entity.id());
+          R"(ignoring unsupported "vehicle" field (tag={}, id={}))", tag, entity.id());
       ++stats.unsupported_vehicle_;
-      continue;
+      // no continue here so we don't skip as long as it also has a trip_update
     } else if (!entity.has_trip_update()) {
       log(log_lvl::error, "rt.gtfs.unsupported",
-          "unsupported no trip update (tag={}, id={})", tag, entity.id());
+          R"(unsupported: no "trip_update" field (tag={}, id={}), skipping message)", tag, entity.id());
       ++stats.no_trip_update_;
       continue;
     } else if (!entity.trip_update().has_trip()) {
       log(log_lvl::error, "rt.gtfs.unsupported",
-          "unsupported no trip in trip update (tag={}, id={})", tag,
+          R"(unsupported: no "trip" field in "trip_update" field (tag={}, id={}), skipping message)", tag,
           entity.id());
       ++stats.trip_update_without_trip_;
       continue;
     } else if (!entity.trip_update().trip().has_trip_id()) {
       log(log_lvl::error, "rt.gtfs.unsupported",
-          "unsupported trip without trip_id (tag={}, id={})", tag, entity.id());
+          R"(unsupported: no "trip_id" field in "trip_update.trip" (tag={}, id={}), skipping message)", tag, entity.id());
       ++stats.unsupported_no_trip_id_;
       continue;
     } else if (entity.trip_update().trip().schedule_relationship() !=