Chapter 5, Http service description of Futures corrected

Fixing prose in Chapter 5 subsection "Introducing the Http service"
since:
- the description of the behavior of `then()` was not quite right;
- described error handling in terms of `catchError()` so as to match
the updated tutorial code.
Merge pull request #10 from chalin/patch-11-chap-05-http-svc-desc-fixed

Chapter 5, Http service description of Futures corrected