Pass error to the error builders?

[hacker1024]

At the moment, PagingController.error is only used in a null check.
It'd be great if this value could be passed into the error builders, as it's cleaner than having to fetch it from the original source in the build function.

[EdsonBueno]

In the earliest versions of the package, the error used to be included in the error builders.
I changed it because of a few points:

• People found it confusing: What's the difference between the error in the builder and the one in the PagingController?
• Should we include the PagingController.retryLastRequest there as well?
• What about the noItemsFoundIndicatorBuilder? It certainly doesn't make sense to provide an error there, but what about the retryLastRequest callback?

That all being said, having the PagingController as the single source is the best/cleaner choice. It keeps the API minimal!

I'm closing this issue. Thank you!