Include code source location in message for competing LoggerFactory impl

To make it easier to identify the source of the competing LoggerFactory
implementation, this commit updates the exception message to include
the code source location of the offending class.

Closes gh-1630

Author: wilkinsona

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

@@ -90,10 +90,16 @@ public void initialize(String configLocation) {
 		Assert.notNull(configLocation, "ConfigLocation must not be null");
 		String resolvedLocation = SystemPropertyUtils.resolvePlaceholders(configLocation);
 		ILoggerFactory factory = StaticLoggerBinder.getSingleton().getLoggerFactory();
-		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() + ")");
+		Assert.isInstanceOf(
+				LoggerContext.class,
+				factory,
+				String.format(
+						"LoggerFactory is not a Logback LoggerContext but Logback is on "
+								+ "the classpath. Either remove Logback or the competing "
+								+ "implementation (%s loaded from %s).",
+						factory.getClass(), factory.getClass().getProtectionDomain()
+								.getCodeSource().getLocation()));
+
 		LoggerContext context = (LoggerContext) factory;
 		context.stop();
 		context.reset();