HTTP 103 not handled

Example request (note it occasionally works but mostly not for me)

```
(plz 'get "https://ben.page/eink")
```

Response:

```
(plz-http-error "HTTP error" #s(plz-error :curl-error nil :response #s(plz-response :version 2 :status 103 :headers ((link . "; rel=preconnect")) :body "HTTP/2 200 \15\ndate: Sat, 13 Jan 2024 10:03:02 GMT\15\n...") :message nil))
```
```
HTTP/2 103
link: ; rel=preconnect

HTTP/2 200
date: Sat, 13 Jan 2024 10:03:02 GMT
content-type: text/html; charset=utf-8
access-control-allow-origin: *
cache-control: public, max-age=0, must-revalidate
link: ; rel="preconnect"
referrer-policy: strict-origin-when-cross-origin
x-content-type-options: nosniff
report-to: {"endpoints":[{"url":"https:\/\/a.nel.cloudflare.com\/report\/v3?s=UIoTEf095%2BCIM55J3%2BP4Crb%2BTLTruAEwDdUgob%2Fs9WQQ%2Bldw8sZ150z50YrdM8%2BVAI9nor8PazOOdVDEO7OjZfhgJBQTS5PLymFdIRVnU8gN9bGouJGr5pOJqg%3D%3D"}],"group":"cf-nel","max_age":604800}
nel: {"success_fraction":0,"report_to":"cf-nel","max_age":604800}
vary: Accept-Encoding
cf-cache-status: DYNAMIC
server: cloudflare
cf-ray: 844cc212ac89108f-HKG
content-encoding: gzip
alt-svc: h3=":443"; ma=86400