feature: error-checking on the request

[ijlyttle]

I was having trouble to get the "versions" test working - so I had box_previous_versions() print out the response object:

List of 6
 $ type      : chr "error"
 $ status    : int 403
 $ code      : chr "forbidden"
 $ help_url  : chr "http://developers.box.com/docs/#errors"
 $ message   : chr "User needs to upgrade"
 $ request_id: chr "a9n6q2g0h15edela"

This is a longer-term "thing", but it would be nice to have a single function that makes requests of the API, checks the response, then returns the response to the higher-level function.