Include stack trace in log file when template engine can't be found

[jonbullock]

Relates to #602

Stack trace should be output in a log file somewhere even if it's not logged to the console.

[pzygielo]

Please, consider changing level used in l.82 (to info or perhaps better - to debug) in

jbake/jbake-core/src/main/java/org/jbake/template/TemplateEngines.java

Lines 74 to 85 in 4007fd2

	private static AbstractTemplateEngine tryLoadEngine(final JBakeConfiguration config, final ContentStore db, String engineClassName) {
	try {
	@SuppressWarnings("unchecked")
	Class<? extends AbstractTemplateEngine> engineClass = (Class<? extends AbstractTemplateEngine>) Class.forName(engineClassName, false, TemplateEngines.class.getClassLoader());
	Constructor<? extends AbstractTemplateEngine> ctor = engineClass.getConstructor(JBakeConfiguration.class, ContentStore.class);
	return ctor.newInstance(config, db);
	} catch (Throwable e) {
	// not all engines might be necessary, therefore only emit class loading issue with level warn
	LOGGER.warn("Template engine not available: {}", engineClassName);
	return null;
	}
	}

As current issue is continuation of #602, I'm not sure if this deserves separate one or could be covered here.

In my maven project following is reported

[INFO] --- jbake-maven-plugin:0.3.4:generate (build-site) @ xxx ---
[WARNING] Template engine not available: org.jbake.template.GroovyMarkupTemplateEngine
[WARNING] Template engine not available: org.jbake.template.PebbleTemplateEngine
[WARNING] Template engine not available: org.jbake.template.GroovyTemplateEngine
[WARNING] Template engine not available: org.jbake.template.ThymeleafTemplateEngine
[WARNING] Template engine not available: org.jbake.template.JadeTemplateEngine
[INFO] Baking has started...

as I have only freemarker added as dependency. (No other engines are needed.) Thus above warnings.

To get rid of them I could add the remaining engines, but I don't want to have unused dependencies to maintain.
If I have no required engine - I expect the error, and that's the case once I remove freemarker.

That's why I think it shall be either info, if it is so important, or rather debug only - to be used in investigation when actually needed engine can't be found. Not always.