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