After executing curl request we need to process curl_request code. It
might be CURLE_WRITE_ERROR. We had special case for it, which assumed
diagnostic message being set and contained corresponding assert, though
it is incorrect. Better way is to handle it as any other non-standard
event.

It was discovered while adding accept_encoding option. In case of
unknown encoding curl_request code is currently set to CURLE_WRITE_ERROR
and therefore we come to an assert assuming we have some diagnostics
set. However, it is not being set and it is totally fine. This means we
are failing on assert and it is not correct behavior.

Prerequisites: #4232

Reviewed-by: Alexander Turenko <alexander.turenko@tarantool.org>