Show error page when runtime exception happens

app/src/main/java/it/chalmers/gamma/adapter/primary/web/GammaErrorController.java

@@ -0,0 +1,29 @@
+package it.chalmers.gamma.adapter.primary.web;
+
+import jakarta.servlet.http.HttpServletResponse;
+import org.springframework.boot.web.servlet.error.ErrorController;
+import org.springframework.stereotype.Controller;
+import org.springframework.web.bind.annotation.GetMapping;
+import org.springframework.web.bind.annotation.RequestHeader;
+import org.springframework.web.servlet.ModelAndView;
+
+@Controller
+public class GammaErrorController implements ErrorController {
+
+    @GetMapping("/error")
+    public ModelAndView handleRuntimeException(@RequestHeader(value = "HX-Request", required = false) boolean htmxRequest, HttpServletResponse response) {
+        response.addHeader("HX-Retarget", "body");
+        response.addHeader("HX-Reswap", "innerHTML");
+
+        ModelAndView mv = new ModelAndView();
+        if (htmxRequest) {
+            mv.setViewName("pages/error");
+        } else {
+            mv.setViewName("index");
+            mv.addObject("page", "pages/error");
+        }
+
+        return mv;
+    }
+
+}

app/src/main/resources/templates/pages/error.html

@@ -0,0 +1,14 @@
+<header th:replace="~{common/header}"></header>
+<main>
+    <article>
+        <header>
+            500 - Internal Server Error
+        </header>
+        <p>Something went wrong...</p>
+        <footer>
+            <a href="/">
+                <button type="button">Go home</button>
+            </a>
+        </footer>
+    </article>
+</main>