feat: add reducer for errors

[thmsobrmlr]

When fetching things from the api, we'd like to display error states. This adds a reducer to make the errorObject available for a loader.

[mariusandra]

Nice. I think this could work... The reason this wasn't done already (despite several false starts) is:

• Some consider storing non-jsonable structures in Redux to be yuk. This will now start putting Error objects in there.
• I was wondering if we should just store error.message or be somewhat clever and never made a final decision.
• Some people might have their own *Error reducers, and this will break things for them.
  ---> so I did got paralysed with overanalysis and did nothing.

However, yes, let's get this in.

I think the first two points are rather moot at the end of the day, but the last bullet needs to somehow be addressed. It might be useful to introduce a plugin option to selectively add the error objects, and leave it turned off for the v3.x versions.

The other question is around typing the error object. This will require some changes to kea-typegen/src/visit/visitLoaders.ts.

An alternative way is to export two functions: loaders() and loadersWithErrors(), and only make the changes in the new one... 🤔 or then have a magic __saveError: true key... 🤔