#It gives an error message in the chat saying that an entity is either immune or a boss and the spell completely fails.

Why doesn't it just skip the immune entity?

I have the target for the spell set to add the caster to a list of nearby enemies. If I get too close to a boss, the spell refuses to work at all. Even if it isn't that close, since the Ender Dragon apparently has a massive hitbox even beyond what's visible.

Spells target any entity at all. Including those that might be immune such as bosses. I haven't heard of anything about Mass Exodus completely failing the moment it targets an immune entity. Just add an error suppression to the spell and it should work just fine.