Improve exception message to hint that you might need a '-spring' file

See gh-42405

Author: nosan

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/logging/log4j2/SpringEnvironmentLookup.java

@@ -43,7 +43,10 @@ public String lookup(LogEvent event, String key) {
 
 	@Override
 	public String lookup(String key) {
-		Assert.state(this.environment != null, "Unable to obtain Spring Environment from LoggerContext");
+		Assert.state(this.environment != null,
+				"Unable to obtain Spring Environment from LoggerContext. "
+						+ "This can happen if your log4j2 configuration filename does not end with '-spring' "
+						+ "(for example using 'log4j2.xml' instead of 'log4j2-spring.xml')");
 		return this.environment.getProperty(key);
 	}
 

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/logging/log4j2/SpringEnvironmentLookupTests.java

@@ -69,7 +69,9 @@ void lookupWhenNoSpringEnvironmentThrowsException() {
 		this.loggerContext.removeObject(Log4J2LoggingSystem.ENVIRONMENT_KEY);
 		Interpolator lookup = createLookup(this.loggerContext);
 		assertThatIllegalStateException().isThrownBy(() -> lookup.lookup("spring:test"))
-			.withMessage("Unable to obtain Spring Environment from LoggerContext");
+			.withMessage("Unable to obtain Spring Environment from LoggerContext. "
+					+ "This can happen if your log4j2 configuration filename does not end with '-spring' "
+					+ "(for example using 'log4j2.xml' instead of 'log4j2-spring.xml')");
 	}
 
 	private Interpolator createLookup(LoggerContext context) {