Add Cookbook S01-S22 (draft)

docs-src/pages/en/cookbook/s13-error-handler.md

@@ -0,0 +1,51 @@
+---
+title: "S13. Return a Custom Error Page"
+order: 32
+status: "draft"
+---
+
+To customize the response for 4xx or 5xx errors, use `set_error_handler()`. You can replace the plain default error page with your own HTML or JSON.
+
+## Basic usage
+
+```cpp
+svr.set_error_handler([](const httplib::Request &req, httplib::Response &res) {
+  auto body = "<h1>Error " + std::to_string(res.status) + "</h1>";
+  res.set_content(body, "text/html");
+});
+```
+
+The error handler runs right before an error response is sent — any time `res.status` is 4xx or 5xx. Replace the body with `res.set_content()` and every error response uses the same template.
+
+## Branch by status code
+
+```cpp
+svr.set_error_handler([](const httplib::Request &req, httplib::Response &res) {
+  if (res.status == 404) {
+    res.set_content("<h1>Not Found</h1><p>" + req.path + "</p>", "text/html");
+  } else if (res.status >= 500) {
+    res.set_content("<h1>Server Error</h1>", "text/html");
+  }
+});
+```
+
+Checking `res.status` lets you show a custom message for 404s and a "contact support" link for 5xx errors.
+
+## JSON error responses
+
+For an API server, you probably want errors as JSON.
+
+```cpp
+svr.set_error_handler([](const httplib::Request &req, httplib::Response &res) {
+  nlohmann::json j = {
+    {"error", true},
+    {"status", res.status},
+    {"path", req.path},
+  };
+  res.set_content(j.dump(), "application/json");
+});
+```
+
+Now every error comes back in a consistent JSON shape.
+
+> **Note:** `set_error_handler()` also fires for 500 responses caused by exceptions thrown from a route handler. To get at the exception itself, combine it with `set_exception_handler()`. See S14. Catch Exceptions.