Improve diagnostics when ServletContext.addListener fails

The various servlet containers that we support vary in the quality
of their diagnostics when ServletContext.addListener fails. To make
problem diagnosis easier, this commit ensures that the toString of the
the listener that was being added is included in the exception that's
thrown when a failure occurs.

Closes gh-2197

spring-boot/src/main/java/org/springframework/boot/context/embedded/ServletListenerRegistrationBean.java

@@ -105,8 +105,14 @@ public class ServletListenerRegistrationBean<T extends EventListener> extends
 			logger.info("Listener " + this.listener + " was not registered (disabled)");
 			return;
 		}
+		try {
 			servletContext.addListener(this.listener);
 		}
+		catch (RuntimeException ex) {
+			throw new IllegalStateException("Failed to add listener '" + this.listener
+					+ "' to servlet context", ex);
+		}
+	}
 
 	public T getListener() {
 		return this.listener;