This topic has been opened in response to a comment from @Old Ben.
It explains the different kinds of errors you'll get from using this mod.
In a gist: Don't worry too much about it, but if you're interested in the technical side of Stellaris modding, I wrote a bit about it down below.
I've taken extensive care that everything works as smooth as possible.

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1. Most errors are due to how compatibility between mods (or with vanilla) is achieved. If you do not subscribe to Planetary Diversity for example, you will get errors about unknown planet classes (yes, just about 500 of them sounds right), and if you do subscribe, you will get their suite of errors instead (ask them about it, if you want). These errors do not matter, because they (at least in my mod's case) won't be triggered in game (and thus won't lead to problems when playing). For example, the code to terraform to Planetary Diversity planet classes exists, but it won't be executed, if you do not have a planet of those classes in the first place. This is why some other modders create additional "compatibility mods" you'd need to subscribe to.

2. Errors due to how modded content goes beyond the vanilla scope. This is the case for the "Eldest Sibling" origin, which requires the re-implementation of a scripted effect that creates pre-FTL empires. However, this effect wasn't done properly in vanilla and thinks that e.g. "origin_mechanists" isn't a real origin, which it most definitely is.

3. Errors due to how specific effects break the engine assumptions. This is the case for "Endeavour to Enlightenment" which cannot check all corner cases of already acquired traits and thus uses a loop to evaluate at runtime which trait to randomly assign. This works against the engine intentions of pre-compiling the results and prints an error warning about "potentially undefined behaviour". It isn't undefined, you just cannot know which trait to assign, without having an already existing leader to work with.

Mods without errors only add new content and don't interact with other mods, or vanilla game play (mechanics). You can write this kind of mod only if your content doesn't rework anything. This is certainly possible (depending on the mod's goals), but this mod can't do that. As I explained, it is not a problem.
Generally speaking: The error.log is mostly relevant only to modders when developing, or when your game crashes as a player and you want to be helpful and send us modders the error (in this case I'd assume either an update broke something, or the mod couldn't be tested perfectly because it is big enough that there's bound to be some kind of oversight). Other than that it could only be helpful to you, if you subscribed to two mods that do the same kind of thing and you need to evaluate which of the mods you might want to request a feature or fix of.
I'd advise that errors which get triggered during gameplay (and even worse if it happens monthly or otherwise regularly) are the only ones you should pay attention to. Any "game start" errors are mostly just warnings.

As a modder I can gate content behind DLCs and we're encouraged to do so where it makes sense (e.g. origins that you can only play using an empire that requires a specific DLC should also be gated behind that DLC). However, not having a DLC won't lead to errors, because the game engine knows about everything anyways (this is because AI empires can use DLC content freely, and if you play a multiplayer session, everyone can use their own DLCs as well as any the host owns).

I hope this has been helpful or interesting to some of you.