apiserver: refactor part 1

[rogpeppe]

No semantic changes as yet.

We standardise on a single sendError method, losing sendExistingError.
We consolidate the four slightly different versions of sendJSON
into a single implementation in httpContext.
We move some code so that type definitions are closer to their methods.

Unfortunately we cannot standardise on a single implementation of
sendError because there are at least four different and incompatible
ways of returning an error from an HTTP request, but we will
attempt to improve things in the next installment.

(Review request: http://reviews.vapour.ws/r/2678/)

[rogpeppe]

$$merge$$

[jujubot]

Status: merge request accepted. Url: http://juju-ci.vapour.ws:8080/job/github-merge-juju