Add instanceof check and nice fat error message

If Logback and another SLF4J implementation are both on the classpath
it is possible for the LogbackLoggingSystem to fail to locate
a LoggerContext. Rather than a ClassCastException it is better to
make an assertion with an error message.

Fixes gh-162

spring-boot/src/main/java/org/springframework/boot/logging/logback/LogbackLoggingSystem.java

@@ -77,7 +77,11 @@ public class LogbackLoggingSystem extends AbstractLoggingSystem {
 		Assert.notNull(configLocation, "ConfigLocation must not be null");
 		String resolvedLocation = SystemPropertyUtils.resolvePlaceholders(configLocation);
 		ILoggerFactory factory = StaticLoggerBinder.getSingleton().getLoggerFactory();
-		Assert.isInstanceOf(ILoggerFactory.class, factory);
+		Assert.isInstanceOf(
+				LoggerContext.class,
+				factory,
+				"LoggerFactory is not a Logback LoggerContext but Logback is on the classpath. Either remove Logback or the competing implementation ("
+						+ factory.getClass() + ")");
 		LoggerContext context = (LoggerContext) factory;
 		context.stop();
 		try {