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 Server Error
Exception: Failed to JSON encode value (#5: Malformed UTF-8 characters, possibly incorrectly encoded): Dictionary value at key "result" is not valid UTF8, and cannot be JSON encoded: diff --git a/latin1.txt b/latin1.txt
new file mode 100644
index 0000000..ce6c927
--- /dev/null
+++ b/latin1.txt
@@ -0,0 +1 @@
+<�>
. at [<phutil>/src/future/http/BaseHTTPFuture.php:339]
```

Reviewers: chad, avivey

Reviewed By: avivey

Maniphest Tasks: T11524

Differential Revision: https://secure.phabricator.com/D16457

src/aphront/response/AphrontResponse.php

@@ -162,7 +162,7 @@ abstract class AphrontResponse extends Phobject {
       $object,
       array(__CLASS__, 'processValueForJSONEncoding'));
 
-    $response = json_encode($object);
+    $response = phutil_json_encode($object);
 
     // Prevent content sniffing attacks by encoding "<" and ">", so browsers
     // won't try to execute the document as HTML even if they ignore