Added APIs :
* httpd_resp_send_err() : for sending HTTP error responses for error codes given by httpd_err_code_t. It uses TCP_NODELAY option to ensure that HTTP error responses reach the client before socket is closed.
* httpd_register_err_handler() : for registering HTTP error handler functions of type httpd_err_handler_func_t.
The default behavior, on encountering errors during processing of HTTP requests, is now to send HTTP error response (if possible) and close the underlying socket. User configurable handlers can be used to override this behavior for each error individually (except for 500 Internal Server Error).
Also fixed some typos.
Closes https://github.com/espressif/esp-idf/issues/3005
* http_sess_set_*_override APIs are now the only ones available to set custom recv/send/pending functions
* Fixed side effects to using http_sess_set/get_context inside URI handlers
