Improve diagnostics for start failures in TomcatStarter

Closes gh-5934

Author: wilkinsona

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

@@ -60,7 +60,8 @@ public void onStartup(Set<Class<?>> classes, ServletContext servletContext)
 			// Prevent Tomcat from logging and re-throwing when we know we can
 			// deal with it in the main thread, but log for information here.
 			if (logger.isErrorEnabled()) {
-				logger.error("Error starting Tomcat context: " + ex.getClass().getName());
+				logger.error("Error starting Tomcat context. Exception: "
+						+ ex.getClass().getName() + ". Message: " + ex.getMessage());
 			}
 		}
 	}