Improve parachains inherent error reporting

[sandreim]

@skunert recently proposed:

One thing we could also look into is better error reporting using this.

We can actually mark inherents as mandatory, then we wouldn't need to do these checks in on_finalize. You can even implement proper errors this way, but a quick play session gives me already:

2025-09-02 15:14:20 Error with block built on 0x2575638eedcbb1657f3b2fe3b3f6828d0afc20d92a825b57a181ad19117f646f: Checking inherents unhandled error: parachn0    

[skunert]

I looked a bit into improving the error reporting for inherents, since both parachain-system and the relay chain inherent use this pattern of checking a storage item in on_finalize.

However, after some research I found the idea to use is_inherent_required not so attractive anymore. I don't see that it brings anything useful to the table. It lets block authoring finish instead of aborting and only aborts at the import stage. I was originally thinking of dumping the block in such cases, but that does not make sense because the inherent will of course not be included. I think we can abandon this approach.