Improve logging from webhook

[QuLogic]

When an error occurs in the webhook (either with itself or due to invalid input), it returns a 4xx/5xx and some error text. For security reasons, Caddy reduces all of these results to a simple 400/503. Since aiohttp only logs the response code, and Caddy only logs admin problems, this information was lost.

We don't really do anything with access logs, so just enable them only for the errors.

Note this is already deployed, and if I request something invalid:

$ http POST https://do.matplotlib.org/gh/matplotlib.github.com \
    Content-Type:application/json \
    User-Agent:GitHub-Hookshot/foo \
    X-GitHub-Event:push \
    X-GitHub-Delivery:foo \
    X-Hub-Signature-256:bar \
    -v < /dev/null

then in the Caddy journal, we get:

Feb 13 09:15:54 mercury.matplotlib.org caddy[464235]: {
  "level": "info",
  "ts": 1739438154.6256573,
  "logger": "http.log.access.errors",
  "msg": "handled request",
  "request": {
    "remote_ip": "...",
    "remote_port": "63586",
    "client_ip": "...",
    "proto": "HTTP/2.0",
    "method": "POST",
    "host": "do.matplotlib.org",
    "uri": "/gh/matplotlib.github.com",
    "headers": {
      "Cf-Visitor": ["{\"scheme\":\"https\"}"],
      "Cdn-Loop": ["cloudflare; loops=1"],
      "Cf-Ray": ["..."],
      "Cf-Ipcountry": ["CA"],
      "Content-Length": ["0"],
      "X-Github-Event": ["push"],
      "Cf-Connecting-Ip": ["..."],
      "Accept-Encoding": ["gzip, br"],
      "X-Hub-Signature-256": ["bar"],
      "X-Github-Delivery": ["foo"],
      "Content-Type": ["application/json"],
      "Accept": ["application/json, */*;q=0.5"],
      "User-Agent": ["GitHub-Hookshot/foo"],
      "X-Forwarded-For": ["..."],
      "X-Forwarded-Proto": ["https"]
    },
    "tls": {
      "resumed": false,
      "version": 772,
      "proto": "h2",
      "server_name": "do.matplotlib.org"
    }
  },
  "bytes_read": 0,
  "user_id": "",
  "duration": 0.232858129,
  "size": 0,
  "status": 400,
  "resp_headers": {
    "Server": ["Caddy"],
    "Alt-Svc": ["h3=\":443\"; ma=2592000"]
  },
  "api_error_text": "400 foo: webhook signature was invalid",
  "api_error_code": 400
}