CCBC-156 Custom handling for OBSERVE during packet errors

The ep-engine renders meaningful body for observe responses only if
status code is 0 (PROTOCOL_BINARY_RESPONSE_SUCCESS). We shouldn't
interpret response body in other cases, just decode & failout request
instead.

Change-Id: Ic1e0ea5d8ac5e7b44f20e0ab79c6ddc58b5f758b
Reviewed-on: http://review.couchbase.org/23570
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>