Rethrow makes sure that any exception in your promise chain won't
swallowed:

getJSON('/posts.json')
    .then(handlePost)
    .fail(RSVP.rethrow)

You can also to chain it further, like so:

getJSON('/posts.json')
    .then(handlePost)
    .fail(RSVP.rethrow)
    .then(null, handleRejection)

The original explanation's 'even though' is misleading, giving you the impression that if the first `.then` included an error handler, the second `.then`'s error handler would have fired, which only the case if the first error handler threw an error or returned RSVP.reject.

Perhaps we'll want another paragraph explaining how to "rethrow" a reject, but for now this should do.

Fix typo in README