Polish "Report friendly error when failing to find AOT initializer"

See gh-38188

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/SpringApplication.java

@@ -437,12 +437,10 @@ private void addAotGeneratedInitializerIfNecessary(List<ApplicationContextInitia
 					initializers.stream().filter(AotApplicationContextInitializer.class::isInstance).toList());
 			if (aotInitializers.isEmpty()) {
 				String initializerClassName = this.mainApplicationClass.getName() + "__ApplicationContextInitializer";
-				if (!ClassUtils.isPresent(initializerClassName, getClassLoader())) {
-					throw new IllegalArgumentException(
-							"You are starting application with AOT mode but not AOT-processed,"
-									+ " please build your application with AOT first."
-									+ " Or remove system property 'spring.aot.enabled=true' to run as regular mode.");
-				}
+				Assert.state(ClassUtils.isPresent(initializerClassName, getClassLoader()),
+						"You are starting the application with AOT mode enabled but AOT processing hasn't happened. "
+								+ "Please build your application with enabled AOT processing first, "
+								+ "or remove the system property 'spring.aot.enabled' to run the application in regular mode");
 				aotInitializers.add(AotApplicationContextInitializer.forInitializerClasses(initializerClassName));
 			}
 			initializers.removeAll(aotInitializers);

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/SpringApplicationTests.java

@@ -1383,8 +1383,8 @@ void shouldReportFriendlyErrorIfAotInitializerNotFound() {
 		application.setMainApplicationClass(TestSpringApplication.class);
 		System.setProperty(AotDetector.AOT_ENABLED, "true");
 		try {
-			assertThatIllegalArgumentException().isThrownBy(application::run)
-				.withMessageContaining(AotDetector.AOT_ENABLED);
+			assertThatIllegalStateException().isThrownBy(application::run)
+				.withMessageContaining("but AOT processing hasn't happened");
 		}
 		finally {
 			System.clearProperty(AotDetector.AOT_ENABLED);