Log error-responses as well as 200 responses.
Log error-responses as well as 200 responses.

We were trying to figure out how much load the server could take, and
were making inaccurate conclusions since many of the requests the
server was handling -- and giving a 500 response to -- weren't being
counted!

I omitted error responses before because I didn't want them to mess up
the timing figures; erroring-out can be fast.  So I keep the
error-counts in a separate bucket and ignoring timing info for them.

Auditors: jlfwong

Test Plan:
Ran
   tools/benchmark_ka_component.js ~/khan/webapp/javascript/*-package/*.fixture.* -d100 -n3 | grep REQ
and saw it say things like:
   REQUESTS IN THE LAST SECOND: 8 ok, 2 errors (63ms/78ms/97ms)
   REQUESTS IN THE LAST SECOND: 10 ok, 0 errors (63ms/72ms/195ms)
   REQUESTS IN THE LAST SECOND: 8 ok, 2 errors (65ms/71ms/107ms)
   REQUESTS IN THE LAST SECOND: 9 ok, 1 errors (62ms/70ms/139ms)
   REQUESTS IN THE LAST SECOND: 6 ok, 4 errors (66ms/69ms/212ms)
which is just what I'd expect (10 requests/second, as per the -d flag).

Khan/react-render-server,Khan/react-render-server,Khan/react-render-server