buffet: extract useful error code definitionss into a separate file

Moved json parser error information from http_utils into its own
file, error_codes.h/.cc, where all future common error codes would
go.

BUG=None
TEST=USE=buffet P2_TEST_FILTER="buffet::*" FEATURES=test emerge-link platform2

Change-Id: Ie5457cedc248612ab512a7161b1fc2cb75758ac5
Reviewed-on: https://chromium-review.googlesource.com/209248
Tested-by: Alex Vakulenko <avakulenko@chromium.org>
Reviewed-by: Christopher Wiley <wiley@chromium.org>
Commit-Queue: Alex Vakulenko <avakulenko@chromium.org>
diff --git a/buffet/buffet.gyp b/buffet/buffet.gyp
index 2197989..001d610 100644
--- a/buffet/buffet.gyp
+++ b/buffet/buffet.gyp
@@ -39,6 +39,7 @@
         'dbus_utils.cc',
         'device_registration_info.cc',
         'error.cc',
+        'error_codes.cc',
         'exported_object_manager.cc',
         'exported_property_set.cc',
         'http_request.cc',