Make sure that handlers always return Json::objectValue.

Reviewers: @nbougalis @scottschurr

This is necessary for the "streaming JSON RPC" review upcoming, but it's also useful in its own right.
You could be right.  But the way I read it, the old code actually removed the `"account"` entry by doing an assignment to `result` (which was probably a bug).  By calling `inject_error` you leave the previous fields in place.  But I may be misunderstanding how the code works.