Homestyx hydra

R1:90294713e850

Use phutil_json_encode() in AphrontResponse to raise better errors

Summary:
Ref T11524. This problem was more difficult to diagnose than necessary because we swallow errors silently in `AphontResponse` when emitting JSON responses.

Instead of using `json_encode()`, use `phutil_json_encode()` which throws on failure.

Test Plan:
Old behavior was HTTP 200 with no body.

New behavior is HTTP 500 with this message:

```
[2016-08-26 07:33:59] EXCEPTION: (HTTPFutureHTTPResponseStatus) [HTTP/500] Internal…
Repository: R1 hydra
Commit Date: Aug 26 2016