Make command line help easier to understand (#968)

* Rework command line output
Add a new exit code when no sample is selcted or found
Only show a few hint lines with easy to understand help instead of full help

* Spelling and wording

framework/platform/platform.cpp

@@ -118,8 +118,7 @@ ExitCode Platform::initialize(const std::vector<Plugin *> &plugins)
 
 	if (!app_requested())
 	{
-		LOGE("An app was not requested, can not continue");
-		return ExitCode::Help;
+		return ExitCode::NoSample;
 	}
 
 	create_window(window_properties);
@@ -137,8 +136,7 @@ ExitCode Platform::main_loop()
 {
 	if (!app_requested())
 	{
-		LOGE("An app was not requested, can not continue");
-		return ExitCode::Close;
+		return ExitCode::NoSample;
 	}
 
 	while (!window->should_close() && !close_requested)
@@ -237,6 +235,20 @@ void Platform::terminate(ExitCode code)
 		}
 	}
 
+	if (code == ExitCode::NoSample)
+	{
+		LOGI("");
+		LOGI("No sample was requested or the selected sample does not exist");
+		LOGI("");
+		LOGI("To run a specific sample use the \"sample\" argument, e.g.");
+		LOGI("");
+		LOGI("\tvulkan_samples sample hello_triangle");
+		LOGI("");
+		LOGI("To get a list of available samples, use the \"samples\" argument")
+		LOGI("To get a list of available command line options, use the \"-h\" or \"--help\" argument");
+		LOGI("");
+	}
+
 	if (active_app)
 	{
 		std::string id = active_app->get_name();

framework/platform/platform.h

@@ -44,6 +44,7 @@ namespace vkb
 enum class ExitCode
 {
 	Success = 0, /* App executed as expected */
+	NoSample,    /* App should show help how to run a sample */
 	Help,        /* App should show help */
 	Close,       /* App has been requested to close at initialization */
 	FatalError   /* App encountered an unexpected error */