Add Cookbook C01-C19 (draft)

docs-src/pages/en/cookbook/c17-error-codes.md

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+---
+title: "C17. Handle Error Codes"
+order: 17
+status: "draft"
+---
+
+`cli.Get()`, `cli.Post()`, and friends return a `Result`. When the request fails — can't reach the server, times out, etc. — the result is "falsy". To get the specific reason, use `Result::error()`.
+
+## Basic check
+
+```cpp
+httplib::Client cli("http://localhost:8080");
+auto res = cli.Get("/api/data");
+
+if (res) {
+  // the request was sent and a response came back
+  std::cout << "status: " << res->status << std::endl;
+} else {
+  // the network layer failed
+  std::cerr << "error: " << httplib::to_string(res.error()) << std::endl;
+}
+```
+
+Use `if (res)` to check success. On failure, `res.error()` returns a `httplib::Error` enum value. Pass it to `to_string()` to get a human-readable description.
+
+## Common errors
+
+| Value | Meaning |
+| --- | --- |
+| `Error::Connection` | Couldn't connect to the server |
+| `Error::ConnectionTimeout` | Connection timeout (`set_connection_timeout`) |
+| `Error::Read` / `Error::Write` | Error during send or receive |
+| `Error::Timeout` | Overall timeout set via `set_max_timeout` |
+| `Error::ExceedRedirectCount` | Too many redirects |
+| `Error::SSLConnection` | TLS handshake failed |
+| `Error::SSLServerVerification` | Server certificate verification failed |
+| `Error::Canceled` | A progress callback returned `false` |
+
+## Network errors vs. HTTP errors
+
+Even when `res` is truthy, the HTTP status code can still be 4xx or 5xx. These are two different things.
+
+```cpp
+auto res = cli.Get("/api/data");
+if (!res) {
+  // network error (no response received at all)
+  std::cerr << "network error: " << httplib::to_string(res.error()) << std::endl;
+  return 1;
+}
+
+if (res->status >= 400) {
+  // HTTP error (response received, but the status is bad)
+  std::cerr << "http error: " << res->status << std::endl;
+  return 1;
+}
+
+// success
+std::cout << res->body << std::endl;
+```
+
+Keep them separated in your head: network-layer errors go through `res.error()`, HTTP-level errors through `res->status`.
+
+> To dig deeper into SSL-related errors, see C18. Handle SSL Errors.