Redirect 404s to frontend

Addresses RISDEV-2956

backend/src/main/java/de/bund/digitalservice/ris/caselaw/config/ErrorHandlerConfig.java

@@ -0,0 +1,14 @@
+package de.bund.digitalservice.ris.caselaw.config;
+
+import org.springframework.context.annotation.Bean;
+import org.springframework.context.annotation.Configuration;
+import org.springframework.web.server.WebExceptionHandler;
+
+@Configuration
+public class ErrorHandlerConfig {
+
+  @Bean
+  public WebExceptionHandler forwardingExceptionHandler() {
+    return new ForwardingWebExceptionHandler();
+  }
+}

backend/src/main/java/de/bund/digitalservice/ris/caselaw/config/ForwardingWebExceptionHandler.java

@@ -0,0 +1,25 @@
+package de.bund.digitalservice.ris.caselaw.config;
+
+import java.net.URI;
+import org.springframework.http.HttpStatus;
+import org.springframework.http.HttpStatusCode;
+import org.springframework.web.server.ResponseStatusException;
+import org.springframework.web.server.ServerWebExchange;
+import org.springframework.web.server.WebExceptionHandler;
+import reactor.core.publisher.Mono;
+
+public class ForwardingWebExceptionHandler implements WebExceptionHandler {
+
+  @Override
+  public Mono<Void> handle(ServerWebExchange exchange, Throwable ex) {
+    if (ex instanceof ResponseStatusException) {
+      HttpStatusCode status = ((ResponseStatusException) ex).getStatusCode();
+      if (status == HttpStatus.NOT_FOUND) {
+        exchange.getResponse().setStatusCode(HttpStatus.FOUND);
+        exchange.getResponse().getHeaders().setLocation(URI.create("/404.index"));
+        return exchange.getResponse().setComplete();
+      }
+    }
+    return Mono.error(ex);
+  }
+}