Improve PropertySourceLoader file extension error

Refine the `IllegalStateException` thrown from `PropertySourceLoader`
for unknown extensions to also indicated that folder references must end
in '/'.

Closes gh-17241

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/context/config/ConfigFileApplicationListener.java

@@ -454,7 +454,8 @@ public class ConfigFileApplicationListener implements EnvironmentPostProcessor,
 					}
 				}
 				throw new IllegalStateException("File extension of config file location '" + location
-						+ "' is not known to any PropertySourceLoader");
+						+ "' is not known to any PropertySourceLoader. If the location is meant to reference "
+						+ "a directory, it must end in '/'");
 			}
 			Set<String> processed = new HashSet<>();
 			for (PropertySourceLoader loader : this.propertySourceLoaders) {

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/context/config/ConfigFileApplicationListenerTests.java

@@ -989,7 +989,16 @@ class ConfigFileApplicationListenerTests {
 				"spring.config.location=" + location);
 		assertThatIllegalStateException()
 				.isThrownBy(() -> this.initializer.postProcessEnvironment(this.environment, this.application))
-				.withMessageContaining(location);
+				.withMessageContaining(location)
+				.withMessageContaining("If the location is meant to reference a directory, it must end in '/'");
+	}
+
+	@Test
+	void whenConfigLocationSpecifiesFolderConfigFileProcessingContinues() {
+		String location = "classpath:application.unknown/";
+		TestPropertySourceUtils.addInlinedPropertiesToEnvironment(this.environment,
+				"spring.config.location=" + location);
+		this.initializer.postProcessEnvironment(this.environment, this.application);
 	}
 
 	private Condition<ConfigurableEnvironment> matchingPropertySource(final String sourceName) {