Pass exception and stacktrace to error builders

[itsJoKr]

We should show a different message depending on the error that happened. For example, if the user has no internet "no internet connection", but if the server throws 500 then we want "server error occurred" or a similar way for other errors.

With the current setup, there is no way to differentiate what happened and show an appropriate error message. The try/catch happens inside this package and all the information about exception or stack trace is lost.

Proposition is to change from:

     firstPageErrorIndicatorBuilder: (context) => IDontKnowWhichErrorHappened(),

to

     firstPageErrorIndicatorBuilder: (context, error, stacktrace) => NowIKnowAndCanHanleIt(error, stacktrace),

[itsJoKr]

My mistake there is actually a way to read errors, although I little bit clunky

firstPageErrorIndicatorBuilder : (context) => MyErrorWidget(pagingController.error, context),