[OTFC] Better error http error responses in daemon

Summary: The daemon now returns error in JSON format

Test Plan:
  $ curl -v http://localhost:8088/set?key=123\&value=fwer
  * About to connect() to localhost port 8088 (#0)
  *   Trying ::1... connected
  * Connected to localhost (::1) port 8088 (#0)
  > GET /set?key=123&value=fwer HTTP/1.1
  > User-Agent: curl/7.19.7 (x86_64-redhat-linux-gnu) libcurl/7.19.7 NSS/3.13.1.0 zlib/1.2.3 libidn/1.18 libssh2/1.2.2
  > Host: localhost:8088
  > Accept: */*
  >
  < HTTP/1.1 200 OK
  < Date: Wed, 08 May 2013 08:03:34 GMT
  < Transfer-Encoding: chunked
  < Content-Type: text/plain; charset=utf-8
  <
  * Connection #0 to host localhost left intact
  * Closing connection #0

  $ curl -v http://localhost:8088/set?key=123\&value=
  * About to connect() to localhost port 8088 (#0)
  *   Trying ::1... connected
  * Connected to localhost (::1) port 8088 (#0)
  > GET /set?key=123&value= HTTP/1.1
  > User-Agent: curl/7.19.7 (x86_64-redhat-linux-gnu) libcurl/7.19.7 NSS/3.13.1.0 zlib/1.2.3 libidn/1.18 libssh2/1.2.2
  > Host: localhost:8088
  > Accept: */*
  >
  < HTTP/1.1 406 Not Acceptable
  < Content-Type: text/plain; charset=utf-8
  < Date: Wed, 08 May 2013 08:03:38 GMT
  < Transfer-Encoding: chunked
  <
  {"Error":"Invalid value","ErrorNo":6001}
  * Connection #0 to host localhost left intact
  * Closing connection #0

Reviewers: tommy.testy

Reviewed By: tommy.testy

Differential Revision: http://dev.chetan.io/D37