Allow Boot's Jetty error handler to be overridden

Fixes gh-19520

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/web/embedded/jetty/JettyServletWebServerFactory.java

 /*
- * Copyright 2012-2019 the original author or authors.
+ * Copyright 2012-2020 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -327,9 +327,9 @@ public class JettyServletWebServerFactory extends AbstractServletWebServerFactor
 			ServletContextInitializer... initializers) {
 		List<Configuration> configurations = new ArrayList<>();
 		configurations.add(getServletContextInitializerConfiguration(webAppContext, initializers));
-		configurations.addAll(getConfigurations());
 		configurations.add(getErrorPageConfiguration());
 		configurations.add(getMimeTypeConfiguration());
+		configurations.addAll(getConfigurations());
 		return configurations.toArray(new Configuration[0]);
 	}
 

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/web/embedded/jetty/JettyServletWebServerFactoryTests.java

 /*
- * Copyright 2012-2019 the original author or authors.
+ * Copyright 2012-2020 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -35,8 +35,10 @@ import org.eclipse.jetty.server.SslConnectionFactory;
 import org.eclipse.jetty.server.handler.ErrorHandler;
 import org.eclipse.jetty.server.handler.HandlerCollection;
 import org.eclipse.jetty.server.handler.HandlerWrapper;
+import org.eclipse.jetty.servlet.ErrorPageErrorHandler;
 import org.eclipse.jetty.util.thread.QueuedThreadPool;
 import org.eclipse.jetty.util.thread.ThreadPool;
+import org.eclipse.jetty.webapp.AbstractConfiguration;
 import org.eclipse.jetty.webapp.Configuration;
 import org.eclipse.jetty.webapp.WebAppContext;
 import org.junit.jupiter.api.Test;
@@ -301,4 +303,24 @@ class JettyServletWebServerFactoryTests extends AbstractJettyServletWebServerFac
 		});
 	}
 
+	@Test
+	void errorHandlerCanBeOverridden() {
+		JettyServletWebServerFactory factory = getFactory();
+		factory.addConfigurations(new AbstractConfiguration() {
+
+			@Override
+			public void configure(WebAppContext context) throws Exception {
+				context.setErrorHandler(new CustomErrorHandler());
+			}
+
+		});
+		JettyWebServer jettyWebServer = (JettyWebServer) factory.getWebServer();
+		WebAppContext context = (WebAppContext) jettyWebServer.getServer().getHandler();
+		assertThat(context.getErrorHandler()).isInstanceOf(CustomErrorHandler.class);
+	}
+
+	private static class CustomErrorHandler extends ErrorPageErrorHandler {
+
+	}
+
 }